My iPad 2 is locked up and I can't shut it off or unlock the screen. It has 90% battery. I was using the camera when it locked up and blacked the screen. If I plug and unplug the power cord, the iPad will wake up but it will not unlock and after a few seconds it goes black again. Holding down the power button does nothing. I've tried holding down the power button and menu button at the same time, but that doesn't work either. Help!