Try starting it up in "Safe Mode" and see if it will load. If it does, then you have something on the device you need to remove. Bad app, virus, malware, etc. If it still won't start up, even in "Safe Mode", then you are looking at either a "Hard Reset", which will wipe the device, or having it looked at by a tech (or the manufacturer if under warranty).
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.