Try starting it up in "Safe Mode" and see if it will fully load. If it does, while you will have limited access to things, you will also know there is something on the tablet that is the cause. Bad app, virus, malware,e etc., which you will have to remove. If, however, it won't even start up in "Safe Mode" then you may have to either "Hard Reset" the tablet, or have it looked at for 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" rather than on.