When you moved the library folder, calibre lost track of it and created a new one.
You can switch back to the library in the new location using the Quick Switch dialog (from the "Library" dropdown):
And next time, use that dialog (third option) to move your library.
...
BTW, when you tried adding the metadata file, that wasn't going to work -- as far as calibre knew, you were trying to add a new ebook format! In order to add metadata via an OPF, you need to use the `calibredb` CLI backend, or use a database restore.