Hi Debby Colman,
Welcome to the Microsoft Q&A forum.
I see that Q&A Assist has already shared some general troubleshooting steps, such as reinstalling Outlook and checking for security software interference. Those are reasonable initial checks, so I’d recommend trying them first if you haven’t already.
That said, I’d like to add a bit more context based on your description. Since you’re able to sign in to your Microsoft account on the web, but signing in through Outlook for Windows (and Excel) fails, this suggests the issue may be related to app‑level sign‑in or cached credentials on Windows, rather than an Outlook‑only problem.
To help narrow this down further, could you please share the exact fault message or error code you see when signing in from Outlook (a screenshot is fine, with any personal details hidden)?
In the meantime, you can also check the following:
- In Windows, open Settings > Accounts and confirm that your Microsoft account is listed and shows as signed in without any prompts or warnings. The wording and layout of this page can vary slightly depending on your Windows version, but the key point is to ensure your Microsoft account is properly connected at the Windows level.
- Sign out of Outlook for Windows, fully close the app, restart your PC, and then sign back in using the Microsoft account sign-in option. This helps refresh the sign-in tokens used by Outlook and other Microsoft apps such as Excel.
Once you share the exact error message, I can give more targeted guidance. I hope this helps clarify the next steps.