If nothing you do allows it to turn off, you may well have to wait until the battery runs out completely for it to shut down.
Once it is fully drained, and you can put it on a charger, do not turn it back on right away. Let it stay off for a bit.
And, if the problem started after you installed or updated a file or app, then you may want to start it up in "Safe Mode" and uninstall the file, update, or app. Then restart it to take it out of "Safe Mode".
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.