Outlook Classic will not open.

Close Outlook and run Outlook.exe /resetnavpane first; this resets the local navigation-pane configuration that can prevent Classic Outlook from opening.

1. Reset the navigation pane

1. Press Windows key + R.
2. Enter:

      Outlook.exe /resetnavpane
   

3. Select OK.

There must be a space before /resetnavpane. This removes navigation-pane customizations but does not delete email. ^1^

If Outlook still produces the same error, continue below.

2. Test Outlook without add-ins

1. Press Windows key + R.
2. Enter:

      Outlook /safe
   

3. Select the default profile and choose OK.

If Outlook opens in Safe Mode:

1. Go to File → Options → Add-ins.
2. Beside Manage, select COM Add-ins, then Go.
3. Record or screenshot the enabled add-ins.
4. Clear their checkboxes and select OK. Disable them—do not use Remove.
5. Restart Outlook normally.
6. Re-enable one add-in at a time, restarting Outlook after each, until the failing add-in is identified. ^1^

If Outlook does not open in Safe Mode, create a clean profile.

3. Create a new Outlook profile

Do not delete the existing profile yet.

1. Hold Shift while starting Outlook.
   Alternatively, press Windows key + R and run:

      Outlook.exe /profiles
   

2. In the profile picker, select Options.
3. Optionally enable Prompt for a profile to be used.
4. Select New.
5. Give the profile a new name and add the same email account.
6. Start Outlook and select the new profile. ^2^

If Outlook opens with the new profile, the old profile is the problem. Leave it in place until the new profile has synchronized and any locally stored data has been backed up. Removing a profile deletes its offline cached content. ^1^

4. Repair a local .pst data file

Use this branch if the affected profile contains a locally stored .pst file or reports that its data file cannot be opened.

1. Close Outlook.
2. Back up the existing .pst.
3. Locate and start SCANPST.EXE. Common Office 2016/2019 installations place it under:

      C:\Program Files (x86)\Microsoft Office\root\Office16
   

4. Select Browse, choose the affected .pst, and select Start.
5. If errors are found, select Repair.
6. Restart Outlook with the profile connected to that file.

The tool might need to run more than once, and severely corrupted items might not be recoverable. For an Exchange account, the .ost is an offline cache rather than the authoritative mailbox; Microsoft documents that it can be recreated, but do not delete it unless the mailbox data is safely present on the server. ^3^

Business-account fallback

For Microsoft 365 for business on Windows 10 or later, Microsoft also provides the Classic Outlook startup troubleshooter through Get Help. It must be run on the same computer where Outlook is installed. ^1^

If a newly created profile also fails, provide the Microsoft 365 administrator with the exact error, account type, Windows and Outlook versions, and whether webmail works. That distinguishes a local data/profile problem from a mailbox or server connectivity problem.

References

1. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
2. Create a profile in classic Outlook for Windows
3. Repair Outlook Data Files (.pst and .ost)