Try starting it up in "Safe Mode" and see if it will load. If it does, then you likely have something on the device that is the problem. Bad app, virus, malware, etc., that you will need to remove to be able to use the device normally. Should it not start up even in "Safe Mode", then you may have to either "Hard Reset" it, which will completely wipe the device, or have it looked at for hardware issues.
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.