Press and hold the Power button for 10-15 seconds. Then leave it for 20-30 seconds and see if it restarts.
To REBOOT:
Press and HOLD all three keys for about 10 - 20 seconds: PowerOn, VolUp and VolDown. Start with the PowerButton. Keep holding past the 3-choice options screen. If you hear a shutter type sound, you've taken a Screen Shot and probably missed the PowerButton.