[Zubersoft]

That is incredibly strange. If you open Apple's files application, then open the MobileSheets folder, do you see any of the files and folders in that directory? Or have they all vanished? Did you perform any actions that could have resulted in the files being deleted? MobileSheets would never go through and delete files on its own - the only time it does that is if you select Settings->Other Settings->Clear Library, or if you restore a backup file and check the option to delete existing files. 

If you go to Settings->Other Settings->Find Missing Files, is every file in your entire library listed as missing?

Mike