Maybe you forgot to copy the file metadata.db and metadata_db_prefs_backup.json in the root of your library folder.
If you don't have these two files but all book sub directoriesin your library directory, you can try to make a library maintenance ->restore database. With this option the database will be rebuild from the saved metadata file metadata.opf of each single book directory. Take a look on calibre manual too.
https://manual.calibre-ebook.com/faq...libre-is-blank
Edit: Kovid was faster...
