Try starting it up in "Safe Mode". If it will load, then you probably have a bad app, virus, malware, etc. on the device that you need to remove to be able to use it normally. Should it not load even then, well you may need to "Hard Reset" the device (which will wipe all data) or it could be a hardware issue.
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.