First you need to get it to shut down. If necessary, pull the battery. Once it is completely off, if you pulled the battery be sure you reinsert it, then follow the instructions below.
Then to reset it try the following...
1. Press and hold the "Volume Down" button and the "Power" button, together, for 10-20 seconds.
2. Now when you see the startup screen appear, release the "Power" button, but continue to hold the "Volume Down" button.
3. You should now see the Android menu giving you the option of doing a "Factory Reset" (which leaves some data) or a "Hard Reset" which completely wipes the tablet.
4. Now just use the "Volume Up" and "Volume Down" to select the option you wish to use. Once it is highlight, hit the "Power" button...