Try starting it up in "Safe Mode" and see if it will load. If it does, then you have a bad app, virus, malware, etc., on the device that you need to remove to get it to work correctly. If, however, it won't start up in "Safe Mode" either, then you are looking at either a "Hard Reset" of the tablet or possibly hardware issues.
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"...