Suddenly to download emails, Thunderbird sayw it can't find my "Inbox" folder. ??

I've been using TB for many years, and without warning, when it tries to down load my mail from the Proton mail server, it says it can't find the folder "Inbox". I've tried to "repair" that folder a couple times unsuccessfully. I've restarted the TB program a couple times, but nothing changes. The error shows up on the bottom "instruction line". I've gone through all my settings and find nothing amiss. Not sure where to go from here. -- Just noticed that I had about 80 emails in that Inbox folder that are now missing completely. Normally, I move read emails to there respective folders, and leave important ones in the Inbox for followup. One significant note here, is I'm still using Win 7 premium. So far it hasn't been a problem. Thanks a bunch, Gerald