Try a hard reset on your tab.
Warning : a hard reset will erase all downloaded apps and user data and restores the tab to factory default condition
Here is a link which shows you how to perform a hard reset. Scroll down and try the third (3rd) method shown in the link first (1st) as it may reset your tab without deleting anything . if it doesn't work try the first (1st) method.
Before you try any reset procedure ensure that your tab's battery is charged or has been on charge for what you think is a reasonable time. If you think that it would be low allow 1-2 hrs . Also do not perform a reset with the charger connected.