Try this...
1. Hold "volume up" button for 15-20 Seconds,
2. While you are holding "volume up" button , also 'press and hold' the "power" button, until tablet shuts itself off.
3. Once it shuts off, release "volume up" button, and the release the "power" button.
This should restart the tablet after this (possibly a few times) before restarting it as you normally would.