I would first check to make sure none of the buttons are stuck. Dirt, etc., can get in there and make them stick. If this is not the case, check to make sure the battery is actually charging. Just because you plug it in, doesn't mean it charged. If you can try it in another device and see if it shows as charged.
If none of these fix the problem, then you would have to try a "Hard Reset" of the device.
NOTE: Doing this will clear 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 tablet.
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, until you see...