Close osu!, move the file "osu!.db" outside of your osu! folder, restart the game and let your beatmaps process after you go to the song selection screen.
Just a note, this will mark all of your maps as unplayed.
It's not a hidden file. Make sure you have file extensions enabled too or it might be a bit hard to find. Without file extensions there are going to be a few files just called "osu!"