Try starting up the tablet in "Safe Mode" and see if it will load. If it will, then you probably have something (bad app, virus, malware, etc.) on the device that you need to remove before you can run it normally. If it won't start up even in "Safe Mode" then you may need to either "Hard Reset" the tablet (which will wipe it of all personal data) or even have it repaired.
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.