If it showed up in Songs folder, they're still there. Just that your osu!.db is corrupted after an abnormal game close. Go to song select and press F5, wait a bit for the game to reload the Songs database.
The osu!.db holds the metadata for the game to reach actual maps/songs file. The game doesn't go to search for the files directly. After import or delete a beatmap, the game update that particular db file so it can reuse later.
Your db file is currently corrupted.