Make sure your device has sufficient charge, press and hold the power button for 20 seconds, After the device has completely turned off, press the power button to restart your device. if that doesn't work call amazon kindle support at Phone: 1-866-321-8851 (US) or 1-206-266-0927 (International) http://tvoffersproducts.com/NewKindleFire
Unplug the Kindle from the power adapter and wait for the power LED to glow green for a few seconds. After a couple of seconds, the screen will go off. Turn the power switch on and off while releasing it. Check out http://www.ehow.com/how_7675095_unfreeze-kindle.html.