Sorry, I am a bit late...
A click on the library icon -> choose switch/create library -> first select your library location then check "use the previously existing library at new location" and finish action with Ok button.
After this your library will open correctly.
If this will not showing your library, you can try to rebuild it (
https://manual.calibre-ebook.com/faq...libre-is-blank). calibre is able to rebuild the file metadata.db (make a backup first).
Assumed, you point to the correct library path, click once more the library icon and choose -> Library maintenance -> Restore database.
This action will build a new metadata.db by scanning each book path within the library folder and reading each metadata.opf. This will take a while. Take care that your library is located at your PC and not somewhere else.