A Microsoft file hosting and synchronization service.
I've been complaining for months - and had seen some recent changes which implied they were doing something in the backend.
I have multiple other accounts sharing with me (other family members), and one stopped working 3 months ago, 2 others stopped working last week (as I assumed they made back end changes).
Then today, finally, all accounts are working! :-)
But Microsoft's communication is absolute ****!
They should provide a post about this issue, what it is, what they are doing, advise if there's anything we can do ourselves to help, and provide comms about the timeline for resolution!
I was very close to cancelling the MS family account and signing up with drop-box, or Google-drive.
All I can suggest to folks on here - hang in there - they might fix it one day.
I'm now taking an extra copy to an external hard drive of all shared folders in case it happens again - as web access to 20,000 photos is just not practical.
GOOD LUCK FOLKS!