A couple of questions. Where is your calibre library stored? Local disk? NAS? Cloud Storage?
If you are using local storage, you could try restoring the database. Click on the library icon => Library Maintenance => Restore database. You may want to delete/move the metadata.db file in the root of the library folder before doing this.
Otherwise see:
I am getting errors with my calibre library on a networked drive/NAS? in the calibre FAQs.