Try holding down Option as you launch Music and then choose the library that it should open, normally ~/Music/Music/Music LIbrary.musiclbrary where ~ is your user's home folder. This may help if Music is trying to use a cached version of the library.
Try holding down Option+Cmd as you launch Music to open it in safe mode, which can sometimes resolve odd issues in the library.
If neither of the above help use the option-start-Music method to create a new empty library or restore your library from an earlier date if you have it backed up in Time Machine.
tt2