Try turning it off and then starting it up in "Safe Mode". If it won't turn off for you, you may well have to wait for it to run out of power. Then charge it but leave it off. Once done charging then try starting it in "Safe Mode". If it loads fine in that mode, then there is probably a bad app, virus, malware, etc on the tablet that you will need to remove to resolve the issue. Once removed you should be OK. If, however, it won't start up even in "Safe Mode" then it could 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.