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Solved Error

How to Fix: 'Outlook Cannot Start. Cannot Open the Outlook Window' Error

Quick Fix

Try this first - it fixes the majority of cases in under a minute: 1. Close Outlook completely (check Task Manager for any lingering OUTLOOK.EXE process and End Task it). 2. Press Windows key + R to open the Run dialog. 3. Type outlook.exe /resetnavpane and press Enter (make sure there is a space before the slash). 4. Let Outlook reopen normally. This resets the folder/navigation pane, which is the single most common cause of this error.

What this error means

The message 'Cannot start Microsoft Outlook. Cannot open the Outlook window' appears when Outlook's user interface fails to fully load. In many cases, Outlook fails to launch because of a faulty add-in, a damaged Outlook profile, a corrupted navigation pane, a stuck background process, a damaged PST or OST file, an outdated Office installation, or a temporary Windows issue. Sometimes, the problem may also happen after a Windows update, Microsoft 365 update, antivirus conflict, mailbox sync issue, or a large Outlook data file. Sometimes an extended version of the error appears, adding 'The set of folders cannot be opened' or 'The attempt to log on to Microsoft Exchange has failed,' which points more specifically toward a profile, data file, or Exchange/Microsoft 365 connectivity problem.

Step-by-Step Guide

Fix 1: Close any stuck Outlook processes (Windows)

  1. 1End stuck Outlook processes in Task Manager. Before trying anything else, confirm that no Outlook processes are running in the background, preventing the app from starting anew. Press Ctrl + Shift + Esc to open Task Manager, or right-click the taskbar and select Task Manager. Under Processes, look for Outlook items, and if you see any, right-click it and choose End task, then try launching Outlook again - if Outlook crashed silently earlier, this simple step often brings it back.

Fix 2: Reset the Navigation Pane

  1. 1The navigation pane (the left-hand folder list) is one of the most frequent culprits. Navigation pane customizations are specific to the local Outlook on a computer, so if you use more than one computer, run this fix on each one. Close Outlook fully.
  2. 2Open the Run dialog. Press Windows key + R to open the Run dialog.
  3. 3Reset navigation pane using Run command. In the Open box, type Outlook.exe /resetnavpane, and then choose OK.
  4. 4Delete navigation pane settings file manually. If the switch doesn't seem to work, delete the navigation pane settings file manually: press Windows key + R, type %appdata%\Microsoft\Outlook and press Enter, then delete the .xml file named after your Outlook profile (for example Outlook.xml). Outlook rebuilds it automatically with default settings the next time it starts.

Fix 3: Start Outlook in Safe Mode to test for add-ins

  1. 1Start Outlook in safe mode. Starting Outlook in safe mode actually means loading the app without any add-ins or custom settings - it's a quick way to check if a faulty plug-in or customization is interfering when Outlook tries to open. To run Outlook in safe mode, click on its icon in the Start menu or Windows taskbar while holding the Ctrl key, then in the pop-up window that appears, click Yes to confirm. Alternatively, use the Run dialog and type outlook /safe.
  2. 2Accept default profile and enter credentials. In the Choose Profiles dialog box, accept the default setting of Outlook and choose OK, and if prompted, enter your password and choose Accept.
  3. 3Confirm safe mode with taskbar icon. The Outlook icon on your taskbar includes an exclamation symbol to confirm you are in safe mode.
  4. 4Disable and test add-ins one by one. If Outlook starts fine in safe mode, one of your add-ins is likely to be the problem - try disabling the add-ins one at a time to detect which one is causing the issue. Go to File > Options > Add-ins, set the Manage dropdown to COM Add-ins, click Go, uncheck everything, click OK, then re-enable them one at a time, restarting Outlook after each, until you find the culprit.
  5. 5Check if safe mode opens Outlook. If Outlook does NOT open even in safe mode, the problem is more likely a corrupted profile or data file rather than an add-in - move to the next steps.

Fix 4: Create a new Outlook profile

  1. 1There may be corruption in your current Outlook profile. You can try creating a new profile by going to Control Panel > Mail, clicking Show Profiles, selecting Add, following the prompts to create a new profile, setting it as default, and restarting Outlook. If Outlook starts normally with the new profile, you've identified your old profile as being corrupt and the source of your problem. Back up your data files before removing the old profile, since all offline cached content will be deleted when you choose Remove.

Fix 5: Repair or rebuild a corrupted PST/OST data file

  1. 1Locate PST or OST data files. Outlook stores mail, calendar, contacts, and tasks in a data file that can become corrupted over time. Close Outlook completely, then check which data file you have: .pst (POP/IMAP accounts and archives) or .ost (Microsoft 365, Outlook.com and Exchange accounts). Both are usually in C:\Users\[username]\AppData\Local\Microsoft\Outlook - press Windows key + R and type %localappdata%\Microsoft\Outlook to open that folder. Older .pst files may also be in C:\Users\[username]\Documents\Outlook Files.
  2. 2Repair PST file with Inbox Repair Tool. For a .pst file, run the Inbox Repair Tool (SCANPST.EXE). In Microsoft 365 and Outlook 2016-2024 it is in C:\Program Files\Microsoft Office\root\Office16 (or C:\Program Files (x86)\Microsoft Office\root\Office16 for 32-bit Office). Browse to your .pst file, click Start, then Repair, and run the scan again until it reports no more errors.
  3. 3Recreate OST file by renaming and restarting. For an .ost file, don't try to repair it - Microsoft recommends recreating it, because it is only an offline copy of your mailbox. With Outlook closed, rename the .ost file (for example to .old) or delete it, then start Outlook, which downloads a fresh copy from the server.

Fix 6: Check if another data file or account is the real problem

  1. 1Switch default data file or account. If you use multiple email accounts, startup issues can occur when the program can't access the default PST file - the file itself may be fine, just temporarily unavailable, for example if it's stored on a network share or cloud folder like OneDrive that can't be reached, or if the file was moved or deleted, causing Outlook to refuse to open. If this sounds like your situation, switching the default account by going to Control Panel > Mail > Data Files, choosing another account with a valid data file, and clicking Set as Default can help.

Fix 7: Rule out Exchange/Microsoft 365 service issues

  1. 1Rule out Exchange or Microsoft 365 service issues. If the full error text includes 'The attempt to log on to Microsoft Exchange has failed,' the cause may be server-side rather than local. In some cases, especially with Microsoft 365 accounts, the error can be related to an Exchange Online service issue, so using Outlook on the web (OWA) or the new Outlook for Windows as a workaround can confirm whether your mailbox is reachable at all. If OWA works fine but classic Outlook doesn't, the issue is local to your Outlook installation/profile, not your account.

Fix 8: Update or repair Office, then reinstall as a last resort

  1. 1Install latest Outlook updates. Outdated Outlook applications may develop bugs that impact functionality, so check if your Outlook is patched with the latest updates and install them if not.
  2. 2Repair Office via Settings or Control Panel. If updating doesn't help, repair Office: go to Settings > Apps > Installed apps (Windows 11) or Control Panel > Programs and Features (Windows 10), find Microsoft 365 or Microsoft Office, click the three dots (or select it) and choose Modify (or Change).
  3. 3Run Quick Repair then Online Repair. Choose Quick Repair first and restart your computer. If the error persists, repeat and choose Online Repair instead - it is more thorough but takes longer and needs an internet connection.
  4. 4Reinstall Outlook or Microsoft 365 if needed. Reinstall Outlook or Microsoft 365 only if both repairs fail.

When to seek different help

If none of the above works, especially with the Exchange-related version of this error on a work/school (Microsoft 365) account, this may be a server-side authentication throttling issue that Microsoft has had to address directly. This error message can occur for different reasons, and not every instance is the same issue, but some support cases have traced it to server-side authentication concurrency limits being reached on user mailboxes. In that scenario, the only way to address it is to open a support case from the Microsoft 365 Admin portal - meaning your IT admin needs to escalate it, as it isn't something you can fix on your own machine. Also seek IT/admin help if you're on a managed work computer without local admin rights, since profile and registry-level fixes may be blocked for you.

Platform notes

These steps apply to classic Outlook for Windows (2016, 2019, 2021, 2024, and Microsoft 365 desktop versions). If you are using the new Outlook for Windows, the underlying causes differ, since the new app doesn't rely on local .pst or .ost files, so issues related to corrupted data files don't apply. For the new Outlook, focus on checking for hanging processes in Task Manager, restarting the computer, updating the app, and using Windows Settings to repair or reset it if it still won't open. Mac and mobile Outlook do not use this exact error message or the same profile/PST system, so if you're on those platforms, the fix will differ (typically removing and re-adding the account).

Sources:

Heads up: this guide was drafted with AI assistance from the real sources listed below, and structured by our team for clarity. It may not cover every possible cause — if it doesn't fix your issue, let us know and we'll take a closer look. See how we create our fixes.

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