OneNote's cloud storage fills up inexplicably quickly; I suspect every auto-save creates a separate file. When the cloud storage fills up, notes are saved in cache form on the phone, NOT as local file(s). So when OneNote crashed repeatedly on startup, I tried clearing the cache. Sike, all the notes you've had that weren't synced onto the cloud and stored in local cache, GONE. Forever. You can restitute deleted files...but only if they were saved on a SD card. If not, you have zero recourse.