Try turning it on in "Safe Mode" and see if it will load. If it does, then you probably have something on the tablet that is causing the issue. Bad app, virus, malware, etc., that you will have to remove. If it still won't start up, then you will have to either "Hard Reset" the device, which will wipe everything and reinstall the original software, or take it to a tech to have them look at it.
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.