If it won't turn off they you would need to wait for it to run out of power, then charge it, without turning it on. Once charged, try turning it on in "Safe Mode". If it turns on, then you have something (bad app, virus, malware, etc.,) on it that you need to remove. If it still won't turn on, then it will either need a "Hard Reset" or there are hardware problems. In that case, if it is still under warranty, I would contact the manufacturer.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.