How to Fix: ‘Cannot start Microsoft Outlook. Cannot open the Outlook window’

Quick Fix

Most of the time this is caused by a corrupted navigation pane file, and it takes under a minute to fix:

1. Close Outlook completely (check Task Manager to make sure OUTLOOK.EXE isn't still running).
2. Press Windows key + R to open the Run box.
3. Type outlook.exe /resetnavpane (with a space before the slash) and press Enter.
4. Let Outlook reopen normally. This rebuilds the corrupted layout file without touching your emails.

Step-by-Step Guide

What this error means

This error appears when classic Outlook for Windows cannot load its main window at startup. It typically comes with a follow-up line like 'The set of folders cannot be opened' or 'The attempt to log on to Microsoft Exchange has failed.'

According to Microsoft and multiple support sources, the most common causes are: a corrupted navigation pane settings file, a damaged Outlook profile, a corrupt or oversized data file (.pst/.ost), a conflicting add-in, or (less often) a temporary Exchange Online service issue. As one support source explains, multiple factors can be responsible for this error, including corrupt Outlook data file, corrupt Microsoft Office installation files, corrupt Outlook profile, incorrect account settings, and problematic add-ins.

This guide is for the classic desktop Outlook app on Windows. If you use the new Outlook for Windows, the causes differ since it does not rely on local .pst/.ost files — see Fix 6 below.

Fix 1: Reset the navigation pane

This is the single most effective and fastest fix reported across Microsoft's own support articles and community threads.

1. Close Outlook completely, including any background process in Task Manager.
2. Press Windows key + R to open the Run dialog.
3. Type Outlook.exe /resetnavpane and choose OK. Note the required space before the slash.
4. If prompted, choose your default profile and click OK.
5. If Outlook starts normally, the navigation pane was the cause — you are done. If you use Outlook on more than one computer, run the command on each one, since navigation pane customizations are specific to the local Outlook on a computer.

If the command itself does not fix it, you can manually delete the underlying file: go to %appdata%MicrosoftOutlook, find the .xml file matching your profile name, and delete it — Outlook will recreate it automatically the next time it starts.

Fix 2: Start Outlook in Safe Mode to test for add-ins

1. Press Windows key + R, type outlook /safe, and press Enter (alternatively, hold Ctrl while clicking the Outlook icon).
2. In the Choose Profile dialog, accept the default profile and choose OK.
3. If Outlook opens fine in safe mode, an add-in is very likely the cause. Go to File > Options > Add-ins, set the Manage box to COM Add-ins, click Go, and uncheck all add-ins.
4. Restart Outlook normally. Then re-enable add-ins one at a time, restarting Outlook after each, until the error reappears — that identifies the problem add-in. Disable that one permanently.
5. If Outlook is not able to start even in safe mode, skip to Fix 3.

Fix 3: Repair your Outlook data file (.pst/.ost)

1. Close Outlook completely.
2. Find SCANPST.EXE in your Office install folder, typically C:Program FilesMicrosoft OfficerootOffice16 (or the equivalent x86 path).
3. Open it and browse to your data file, usually located at C:Users[username]AppDataLocalMicrosoftOutlook.
4. Click Start to scan, then Repair if errors are found.
5. Reopen Outlook and check if the error is gone.

Fix 4: Create a new Outlook profile

If resetting the navigation pane and repairing the data file don't help, your Outlook profile itself may be corrupted.

1. Open Control Panel > Mail (search 'Mail' in the Windows search bar if you can't find it).
2. Click Show Profiles, then Add.
3. Name the new profile and follow the prompts to add your email account.
4. Under When starting Microsoft Outlook, use this profile, select the new profile (or choose 'Prompt for a profile to be used') and click OK.
5. Start Outlook. If it opens normally, the old profile was corrupt. Back up any local data files before removing the old profile, since all offline cached content is deleted when you choose Remove.

Fix 5: Repair the Office/Outlook installation

1. Go to Settings > Apps > Installed apps (Windows 11) or Control Panel > Programs and Features.
2. Find Microsoft Office or Microsoft 365, click the three dots, and select Modify.
3. Choose Quick Repair first and restart your computer afterward.
4. If the error persists, repeat and choose Online Repair instead, which is more thorough but takes longer and requires an internet connection.

Fix 6: New Outlook for Windows or Microsoft 365/Exchange-specific issues

1. If you use the new Outlook app, it doesn't rely on local .pst/.ost files, so data file corruption fixes don't apply. Instead, end any hanging Outlook processes in Task Manager, restart your computer, update the app, or use Windows Settings to repair/reset it.
2. Check mailbox storage. A mailbox that is almost full can prevent Outlook from refreshing or loading folders properly and can trigger the same error.
3. Try Outlook on the web (OWA) as a workaround to confirm the mailbox itself is healthy while you troubleshoot the desktop app.
4. If the error specifically mentions 'The attempt to log on to Microsoft Exchange has failed' and none of the above works, this can occasionally be a server-side Exchange Online authentication throttling issue. In that case the only way to address it is to open a support case from the Microsoft 365 Admin portal.
5. Run the Support and Recovery Assistant (SaRA) tool from Microsoft, which can automatically diagnose and repair common Outlook profile and sign-in problems.

When to get further help

1. Business/Microsoft 365 accounts: If your IT admin manages your account and none of these fixes work, contact your organization's helpdesk — the mailbox may need server-side attention.
2. Severely corrupted data file: If scanpst.exe repair fails repeatedly, restoring a previous version of the .pst/.ost file (right-click the file > Properties > Previous Versions) or setting up a fresh profile without importing the damaged file may be the only option.
3. Nothing works: As a last resort, restoring Windows to a point before the error started, or contacting Microsoft Support directly, are viable next steps.