I was having the exact same problem and following Dwanthy's step's restored my library and covers. In my case it was a chronic issue that kept reappearing. Crossing my fingers that it has been resolved.
[QUOTE=dwanthny;908332] I created a step by step process to backup and restore Calibre's books in
this thread.
You may also wish to consider creating a backup system once it is restored. The thread talking about it it is here along with the code examples.
https://www.mobileread.com/forums/sho...t=83311&page=3
I did mine today and it wasn't that challenging (of course that is because I had help and great guidance
). You copy code into batch files and use Task Scheduler.
Good Luck!