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.

Step-by-Step Guide

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a damaged Outlook profile, a corrupted navigation pane, a stuck background process, a damaged PST or OST file, an outdated Office installation, or a temporary Windows issue.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 1: Close any stuck Outlook processes (Windows)

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.

Step 2: Reset the Navigation Pane

The 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.

1. Close Outlook fully.
2. Press Windows key + R to open the Run dialog.
3. In the Open box, type Outlook.exe /resetnavpane, and then choose OK.
4. If the switch doesn’t seem to work, you can instead find and delete the XML file that holds the navigation pane settings by pasting the config folder path into the Run box or Explorer’s address bar, then finding and deleting the file named for your profile with the extension XML. Outlook will rebuild it automatically with default settings.

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

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.

1. 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. 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. The Outlook icon on your taskbar includes an exclamation symbol to confirm you are in safe mode.
4. 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. 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.

Step 4: Create a new Outlook profile

There 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.

Step 5: Repair a corrupted PST/OST data file

Outlook stores mail, calendar, contacts, and tasks in a data file that can become corrupted over time. You can run the Inbox Repair Tool (scanpst.exe) to scan your Outlook data files and repair errors – exit Outlook first to use the tool. On Windows, find SCANPST.EXE inside your Office installation folder (typically under Program FilesMicrosoft Officerootoffice), run it, and browse to your .pst or .ost file, usually located at C:Users[username]AppDataLocalMicrosoftOutlook. Run the scan repeatedly until it reports no more errors.

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

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.

Step 7: Rule out Exchange/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.

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

1. 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. If updating doesn’t help, go to Control Panel > Programs and Features, select Microsoft 365/Office, click Change, and run an Online Repair (more thorough than Quick Repair).
3. Try Quick Repair and Online Repair first, and reinstall Outlook or Microsoft 365 only if repair does not fix the issue.

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).