You may need to reset the device, which, sadly, will cause a loss of any and all data you had on the device.
You have two options for doing it. First, option is a "Factory Reset", which will usually only remove some, but not all, data. To do that, locate the pinhole on the device and using something small (but not sharp, or you could do damage), and depress the button in the pinhole.
If this doesn't work, then you may want to try a "Hard Reset" which will wipe all personal data off the tablet.
NOTE: Doing this will clear 'all' 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 device.
Be sure you back up any data you want to keep, to a...