Try starting the tablet in "Safe Mode" and see if it will boot up. If it will, then you have something (bad app, malware, virus, etc.,) on it that needs to be found and removed. If it won't boot in "Safe Mode" either, then you will need to do a "Hard Reset". Your phone has to be turned OFF at the start, to access either "Safe Mode" or to do a "Hard Reset".
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...