Sometime in the last few days it stopped working. You can see that your account is fine if you log in via a browser, and it works on your phone - but Outlook on your PC suddenly decided you no longer exist.
If you click the error notification in the bottom status bar in Outlook, you see one or more of the following errors:
There is an error synchronizing your mail account. Please verify your account is configured correctly by first accessing your mail on the web. Error: 3219. Server. An error occurred in looking up the user's information in mserv.
Error with Send/Receive.
There was an error synchronizing your folder hierarchy. Error : 80041004.
“Task ‘your email address’ reported error (0x8004102A)
‘Error with Send/Receive’