A Microsoft file hosting and synchronization service.
Yep, it actually did. Mind, I was accessing OneDrive only via my laptop, not via my iPhone or by other means. I was using the Chrome browser, at the time they disappeared, AND when they reappeared. I about had a heart attack when all my files were missing after having reset my MSOutlook password, because although I have them elsewhere, it would be a LOT of work to put everything that I share with students and family back in place. Went to bed stressing over it after several logouts and re-logins and poking around online to find a solution. But upon arising in the morning, getting online ... <bang> there they were, visible again. Go figure. Great for me, but that means I can't suggest anything helpful to anyone else.