You may have to do a hard reset of the device to eliminate the problem. But I would seriously use that only as a last resort, as this will cause you to lose all your personal data.
Should you choose to try a hard reset....
To "Hard Reset" the tablet...
1. Make sure that the tablet is actually powered off.
2. Press the "Power" and the "Volume Up" buttons, at the same time. Hold them down until you see the start-up logo on the screen.
3. Once you can see the logo, release both buttons and allow the device to enter the "System Recovery Mode".
4. Next use the "Volume Down" button to highlight the "wipe data/factory reset" option.
When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
5. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
6. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
7. Then press the "Power" button to hard reset the tablet.
Should you not wish to do the hard reset, then you may wish to take it to a repair center.