Try starting it up in "Safe Mode". If it fully loads (it will have restricted access) then you know there is something on the device causing the problem. Bad app, malware, virus, etc. Then you would need to remove the problem. If that doesn't get you in, you may have to try a full "Hard Reset".
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 steps above, and you should see "Safe Mode: OFF" rather than on.