Prior to March, 2015 upgrade, all my music appeared by Artist. After the March, 2015, upgrade, about half of my music appears by Artist and then there's one giant "unknown" file. Why would this change with the upgrade? Makes my music unsearchable unless I want to search by the song name or album name.
You can try an app like Album Art in the Play Store and it will take any tag info and find and sort music. Should work unless the music files were altered during update, in that case you'd need to remove those files and re-add them to the phone.