Thank you to everyone for your advice and at least I know it wasn't me! LOL anyway, I managed to solve the problem by deleting everything from my calibre library, and reloading it from my backup folder, which I do at least once a month or if I add several books will do a back up. This has really saved my behind! It would have been a HUGE pain to reload everything manually.
I will keep in mind all that everyone wrote in case I have this issue again. Thanks everyone!