Have you tried shutting it down fully (rather than just turning the screen off) then restarting it?
On Android tablets, shutdown is usually achieved by keeping power button pressed for 5 secs..
Failing that, have a look on the back panel for a small hole marked 'Reset' and push a straightened paper-clip in to it while the tablet is running. Keep paper-clip pressed in hole for 5 to 10 seconds before releasing it.