Try turning it on in "Safe Mode" and see if it will load. If it does, then you likely have something on the device that needs to be removed. Bad app, virus, malware, etc. If, however, it won't start up even in "Safe Mode", then you may have to either "Hard Reset" the device (which will totally wipe it and start it over without your personal stuff) or take it to a tech to be looked at. NOTE: If it is under warranty, contact the manufacturer rather than taking it to a tech.
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.