WHERE (drive-folder) is the Library kept?
NAS or Cloud sync's are not friends of Calibre running at the same time.
Stop Sync (Library folders) before you use Calibre.
Do NOT run from a NAS or Share (See Calibre FAQ's)
If you are ONLY on a local drive (most common):
Have you checked you PC Event Viewer logs? Your HD may be having issues (about to belly up. I caught one failing a few years ago and quickly cloned-replaced the drive
Cloning and a drive swap can be done in a couple of hours. A full rebuild restore, days
).