Microsoft out look will not start cannot open window

For classic Outlook on Windows, use this sequence:

1. Start Outlook in safe mode.
   • Press Windows key + R.
   • Type Outlook /safe and select OK.
   • If Outlook opens in safe mode, the problem is likely an add-in.
2. Disable COM add-ins if safe mode works.
   • In Outlook, go to File > Options > Add-ins.
   • At the bottom, confirm Manage shows COM Add-ins, then select Go.
   • Record the currently enabled add-ins.
   • Clear all selected check boxes, then select OK.
   • Close Outlook and start it normally.
   • If Outlook opens, re-enable add-ins one at a time until the failing add-in is identified.
   • Do not use Remove during troubleshooting; disable only.
3. Reset the navigation pane.
   • Close Outlook.
   • Press Windows key + R.
   • Type Outlook.exe /resetnavpane and select OK.
   • This removes navigation pane customizations.
4. Create a new Outlook profile if Outlook still does not start.
   • Create a new profile using the steps in Create an Outlook profile.
   • Restart Outlook.
   • In Choose Profile, select the new profile and choose OK.
   • If Outlook starts normally, the original profile was corrupted.
   • If removing an old profile later, back up data files first. Removing a profile deletes offline cached content.
5. Repair Outlook data files.
   • Exit Outlook.
   • Run the Inbox Repair Tool (scanpst.exe) using the instructions in Repair Outlook Data Files (.pst and .ost).
6. If available, run the classic Outlook startup troubleshooter.
   • The classic Outlook startup troubleshooter in Get Help can diagnose and fix several common startup issues.
   • It requires Windows 10 or higher and must be run on the same PC as classic Outlook.

If the error specifically includes “The attempt to log on to Microsoft Exchange has failed”, use Outlook on the web or new Outlook for Windows as a workaround. For the known Exchange Online mailbox issue described by Microsoft, resolution requires a support case from the Microsoft 365 Admin portal.

References:

• I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
• Error when opening classic Outlook: "Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The attempt to log on to Microsoft Exchange has failed." | Microsoft Support