Your post is not very clear;
1. By having "all my music in a file titled 'music'", are you referring to the name of your MusicBee library, or do you mean all your music audio files in a FOLDER titled music, or what?
2. By "completely disappeared", what do you mean exactly? What happened that makes you think "it" (library or music files) is gone? What did you do or change?
The backup file MusicBee3Settings.bak is a backup of your MusicBee settings; there is also a backup of your MusicBee library (this is NOT your physical music files), MusicBeeLibrary.bak.
If you feel your audio files are gone, did you search your PC using Windows Explorer or File Explorer to see if they are actually in whatever folder(s) you had/have them stored in? If they are indeed GONE, erased from your drive, unless you have a backup (as everyone should) I'm afraid you may be SOL. MusicBee does not delete all your music files or ANY music file unless you tell it to, and only after you confirm a message first asking if you want to delete them from the library AND computer.
If your files are really still there (and I hope they are!) and the issue is you can't see them in your MusicBee library, the first thing I would try is renaming MusicBee3Settings.bak to MusicBee3Settings.ini (after first deleting the existing MusicBee3Settings.ini) and restarting MB. Maybe you just screwed up the settings to make you think the files are gone.
If restoring the settings file backup doesn't work, do the same with the MusicBeeLibrary.bak file, renaming it to MusicBeeLibrary.mbl after deleting the existing MusicBeeLibrary.mbl and restart.
Until you give us more information, those are the only suggestions I have.