You did not tell us which tablet you have ;-) but try this [http://google.about.com/od/socialtoolsfromgoogle/qt/How-To-Reset-A-Frozen-Motorola-Xoom-Tablet.htm|from here] :
"To reset your Xoom when the screen is totally unresponsive, press the power and volume up buttons at the same time for about three seconds. The two buttons are right next to each other on the back and side of your Xoom. This is a soft reset. It's the equivalent of yanking the batteries or totally powering the device off."