A Microsoft file hosting and synchronization service.
Many are having this failure (see cases on my profile) now, yet none of them has reported resolution. One older case from last summer took two months - All Files on OneDrive Disappeared. - Microsoft Community although there was an additional issue with verifying his account ownership to support.
OneDrive support has not been forthcoming about what is going on, expected timelines, who to contact or even if we need to in order to get resolution, etc. My email support said they are "backlogged due to unavoidable circumstances."
Update: all my files are all back and everything is back to normal.