[stumped]

i would blame that out of date looking windows 10 you are running and/or its associated windows defender AV, not calibre.

assuming its not user error, it looks like an over zealous AV or reg. cleaner or some such has removed what it thought was junk / malware.

NB if you are expecting to find your library books in \AppData\Local\Calibre_library.. you are looking in the wrong place, unless you have a very strange non standard install. your books sh\uld be in documents/calibre library. NB If you have allowed microsoft One drive to sync that folder, which it now tries to do by default, you have a new leading suspect...

Create an empty new library then use " add books" to add all the books back again from a backup folder, that should give you a working library