Sony SGPT21 Series Help Manual page 148

Your tablet device does not work.
Press and hold the power button for about 10 seconds to force it to turn off, and then turn it on again [Details].
If you cannot force your tablet device to turn off, remove the battery pack after unplugging the AC adapter,
then install the battery pack in your tablet device again. Turn on your tablet device after a short while you
install the battery pack in your tablet device.
Contact the service/support center named in the warranty when the symptom does not go away.