If you haven't already, turn the tablet off (not sleep) and then back on. If that doesn't work, there are two things to try. One is to do a factory reset (which will remove all of your settings, documents, and appls), and the other is to turn the tablet off, remove the back cover, remove the battery, and then replace the battery and door. Whichever you choose to do depends on how comfortable you are with the various tasks.
If you still can't get it to work, contact customer support at (800) 457-7777.
- Peter