Well you are either going to have to pull the battery (yes I know they are hard to get to) or wait for it to run out of power. Then, once it is off, leave it off and plug it in to charge. Only charge it for a short while and then try turning it on in "Safe Mode". If it works, you will have limited access, but then you can check the device for malware, viruses, bad apps, etc. Things that could cause the problem in the first place. Remove anything found to be a problem and then restart the tablet (turn it off and back on). It should then work fine.
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.