what files store the album ratings?
C:\Users\[USERNAME]\AppData\Roaming\MusicBee\MusicBee3Settings.ini ?
C:\Users\[USERNAME]\Music\MusicBee\MusicBeeLibrary.mbl ?
They are stored in MusicBeeLibrary.mbl, unless you chose to save them in the files themselves (Prefs/Tags(1)/tag storage).
I don't use ratings myself so no idea how they could be lost. If you chose to store them in the files, check them with Tag Inspector to see if they are still there. If you make backups of your music collection it would be easy enough to just restore the backup. The MusicBeeLibrary.mbl file can be copied elsewhere manually or use File->Library->Backup Library. When you change things MB makes its own backup of the library named MusicBeeLibrary.bak--you could try exiting MB, renaming that file's extension it to *.mbl, and restarting. First rename the original's extension to something like *.mbl.orig so you can change back if needed.
Try rescanning all your files, and then see if they show up again.