This happend to my kindle to! After keeping in in flight mode for years I put it online again in order to buy a few new books from the kindle store, poof suddenly my entire library of side loaded books was gone, with progress and everything. I could see random metadata files related to the books on the drive, be books was gone. Super annoying as many of the books I didn't have locally anymore and to loose the "archivement" of finished books sucks big time. I can see this may be implemented by amazon to counter piracy, but alot of these books was perfectly legal. So the result of this is that I will never put my kindle online again and just stop buying from the Kindle store.