Try starting it up in "Safe Mode". If it will load in that, then you probably have a virus, malware or bad app on the tablet that needs to be removed to resolve the problem. If it won't start up, even in "Safe Mode" then you may have to do a "Hard Reset" on the tablet, however save that as a last resort, as it will wipe all your personal data.
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...