Try starting it up in "Safe Mode" and see if it will fully load. If it does, then you have something on the tablet that is interfering. Bad app, virus, malware, etc., that will need to be removed before it can run normally. Now if it still won't start up, even in "Safe Mode", then you can try a "Hard Reset" (just be sure as that will wipe all data) or take it to a tech to have it checked 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.