TL;DR: Classic Outlook stuck on “Loading Profile”: how to get it open without losing mail

The fix only. No explanation, no diagnosis. Read the full article if any of this does not match what you are seeing.

1. Close the hung window: Task Manager (Ctrl+Shift+Esc), end Outlook (OUTLOOK.EXE), then wait a minute.
2. Run Windows Update until it says "You're up to date" (a January 2026 Windows update hung classic Outlook profiles that use POP or PST files; Microsoft says Windows Update fixes it). Restart.
3. Start Outlook in safe mode: press Win+R, run:

outlook.exe /safe

4. If safe mode opens: File > Options > Add-ins, Manage: COM Add-ins, Go, untick all, restart Outlook normally. Re-enable them one at a time to find the culprit.
5. Check what the profile contains before changing anything: Control Panel > Mail (Microsoft Outlook) > Data Files. Note every .pst and .ost and where it lives.

Never delete a PST. It can be the only copy of the mail. If the profile uses POP, or a PST sits in OneDrive, move the PST out of OneDrive first.

6. Try Office repair: Settings > Apps > Installed apps > Microsoft 365 > Modify > Quick Repair. If that does not help, try Online Repair.
7. Exchange or Microsoft 365 mailbox only: close Outlook and rename (do not delete) the .ost.

Rename-Item "$env:LOCALAPPDATA\Microsoft\Outlook\<your address>.ost" "<your address>.ost.old"

Do not do this for a POP or PST-only profile. Any item that exists only in the local file is lost.

8. Last resort: Control Panel > Mail > Show Profiles > Add a new profile, set "Always use this profile", and keep the old one until the new one works.
9. Verify: Outlook opens from the normal shortcut and shows the Inbox.

← Full article, with the diagnosis and the why