open the back up, take out the batteries, and put them back in. try and turn it on.
if that dosentwork, take out the batteries again, and get a very small screwdriver and undo the backup battery. youll see it, its right above where the batteries usually are. unscrew it, take it out, then put it back in. put the rest of the batts back in and itll work.
i did that to my friends calc once on accident, i had to fix it.