Hi
If you plug it in, does it make any noises, or vibrate?
Try:
Press and hold the VOLUME DOWN button, and then press and hold the POWER button.
Wait for the screen with the three Android images to appear, and then release the POWER and VOLUME DOWN buttons.
From that screen, choose the reboot option. If there is no option for that, press and hold power button till it goes off.
If none of this is possible, then I'm afraid you'll have to send in for a repair.