Have you tried just rebooted the tablet? Sometimes that will fix the issue.
Also, if that doesn't work I would suggest trying to restart it in "Safe Mode" as if it is something that was added to the device that is hindering it from working correctly, rather than the tablet actually having physical problems, then a "Safe Boot" may get you around it so you can remove whatever is causing the issue.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable safe mode, simply repeat the steps above, and you should see "Safe Mode: Off" rather than On.