Came across an issue recently where a working Outlook that was linked to a GMail account suddenly stopped working. Doing a bit of research, I stumbled upon this great article directly from Microsoft about fixing the issue.
Apparently the stored credentials got messed up and have to be done over. Following these instructions in the link above will likely fix the issue. If yours isn’t fixed by this, you may have a different problem.
Bear in mind that you may have to remove and re-add the account to Outlook. If you do so and get an error stating “We couldn’t log on to the incoming (POP/IMAP) server” then the above link should help.