Sounds like you may have a corrupted data base. How it happened though is anybody's guess. So if you have a good copy of your data base You can restore it from calibre if you using the extended kobo driver. If not log out on your device and log back in. Your device will rebuild the book data base, wont take too long if you have a small number of books. I have over 5000 ebooks and it can take quite some time on my device, so much in fact I have to plug it in to do the job.
Good luck
Jack
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