A Microsoft file hosting and synchronization service.
My wife and I also got ours back, but differently. In my case, it just came back on its own. In my wife's case she had the shared folder with only some of the subfolders and files. Turns out, it was a replica. We were able to delete it, then create a shortcut from the actual shared folder.
My understanding is that Microsoft is making some architectural changes on the back end, and migrating users is what's breaking this. When the migration is complete, it should just come back, but as my wife's experience showed, sometimes it's not seamless.