Outlook 365 No Longer Works

For some unknown reason, a week and a half ago Outlook 365 suddenly stopped working - it keeps prompting me to enter a username and password, something it hasn't ever done before. I have checked to make sure the settings are correct, I've removed my account, added it back, uninstalled Outlook and reinstalled it - nothing has resolved it. If I use the FREE version of Outlook - email works just fine but I don't want to use the free version of Outlook because I don't want the ads. I've compared the settings between the free version of Outlook and my licensed version and they are identical - so why does my Comcast email work in one application but not the other? I've seen numerous complaints about this in the forums but no answer as to what resolves it.