May I wish you a warm welcome to the CameraLabs forum. Sorry to hear your 5D2 is broken.
If it's still under warranty then there's only one course of action so I guess the fact that you haven't already returned it for repair says that it's no longer covered.
It sounds very much like a hardware failure to me (though I'm no expert) but I guess you could always try a firmware upgrade (v2.0.8 plus instructions is available here
) if the usual trick of taking the battery out for 10 or 15 seconds doesn't help (I doubt it will). The only other thought that occurs to me is whether water may have got into the camera so if you have somewhere warm (not hot) and of relatively low humidity where the camera could spend a day or two drying out then that might also be worth a shot before opting for a potentially expensive repair.