Take it apart according to one of the many youtube videos available, then take a close look at the circuitry. Is anything missing? Is there anything rattling around? Something has proably broken off inside, can probably be superglued back on. Don't panic...if that's no good, go to a small independent gamer's shop (not Game!) and have a quiet word with one of the staff about it, don't admit to openeing the DS until he says it's OK to do so, you may invalidate your warranty if they do everything by the book!
Hugh, February 2009