First you are going to have to get it to shut down. If it isn't under warranty, you can try opening it up (if you feel comfortable doing so) otherwise you will have to wait until it runs out of power. If you wait, then leave it off and plug it in. Then, after it charges for a short while, try starting it up in "Safe Mode" and see if it will fully load. If it does, then you may well have a bad app, malware or virus on the device which will need to be removed for you to be able to use it normally as "Safe Mode" is restricted access, so not all will work while in it.
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.