Try turning it off and then starting it up in "Safe Mode". If the problem is a bad app, virus, malware, etc., it should start up in "Safe Mode". Granted it will have limited access, but should start. If this is the case, you will then need to remove what ever item(s) are the problem and then restart the tablet.
Should it not start up, even in "Safe Mode", then it may be a hardware issue.
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.