Can you get any more details? Like what kind of error it is?
If not I'd suggest the following:
Delete osu!.exe and osu.dll
Run osume.exe
Try running osu! again. If it still doesnt work, delete osu!.cfg and osu.db and see if that helps. You may want to make a backup of your configuration.
Also tell me which of the above fixes it and if it is your configuration, please attach the original to your reply.