Try starting it up in "Safe Mode". If it will work correctly in there, then you have an app, virus, malware, etc., on the device that is keeping you from accessing it normally. If, however, it won't work correctly in "Safe Mode" either, then you may need to "Hard Reset" the device, which would be a last resort, or there is a hardware problem that will need to be looked at.
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 it, simply repeat the...