Usually shutting off and restarting will be all you need but if this gives you no joy then you need to try booting from the service menu. You get there by shutting off the tablet completely and then holding down the power button and the volume down button and the same time. When the android icon comes up and the services list select reboot now. From here it should boot into a normal mode. If you still have no joy this video may help
https://www.youtube.com/watch?v=VM0iej6TyoQ
Hope this helps