Please don't immediately suggest reinstalling. Most of the time doing that doesn't fix anything, and while it can fix some issues, usually there areless destructive things that can be tried before doing that. At the very least, mention that uninstalling the game will remove all of your beatmaps, skins, local replays/scores, etc. And, in order to save all of those things, nearly the entire game needs to be backed up anyway.
In this case, I'd try restarting your computer first.
If that doesn't do anything, try repairing the game. Keep the shift key held down while starting the game until a configuration window pops up. There should be a "repair osu!" button on the window. Alternatively, you may have a "repair osu!" executable already inside your osu! folder, double clicking on that should also open that window.
If neither of those things work, close the game, open your osu! folder, move the "osu!.db" file outside of the folder, and then start the game again.