Try starting it in "Safe Mode" and see if it will load. If it does, then there is something on the device that is the problem. Bad app, virus, malware etc., that will need to be removed.
If, however, it still won't start up, even in "Safe Mode", then you are looking at either a fully "Hard Reset" to fix the problem (if software related). This will wipe the device completely and start it over as it originally was when you first got it. Or it will need to be looked at for hardware problems.
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...