Well you can try waiting for the battery to drain completely to have it turn off on its own, or you can try doing a "Hard Reset" of the device, which I will include instructions on how to do that. That said, if the device is new, or still under warranty, you should contact the manufacturer about the issue.
NOTE: Doing this will clear 'all' the data on your tablet, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the tablet...
1. Make sure that the tablet is actually powered off.
2. Press the "Power" and the "Volume Up" buttons, together...