Well you are going to have to turn it off to do anything. If you can't turn it off on your own, then you will either have to wait for it to drain of power or pull the battery.
Yes, I know tablet batteries are hard to get to. If your device is under warranty then don't open it up, however if it isn't then that is another option for getting it to turn off.
Once off, let it charge for a little while (not fully) and then try turning it on in "Safe Mode". If it will turn on then, which is a restricted mode, then you probably have a virus, malware or bad app on the device that needs to be removed. Once removed you can restart the tablet to get out of "Safe Mode".
If it won't start in "Safe Mode" either, then you may well have to do a "Hard...