Try starting up the tablet in "Safe Mode" and see if it will load. If it will, then you have something on the tablet that needs to be removed. Virus, malware, bad app, etc. If it still won't start normally, even in "Safe Mode" then you will either have to "Hard Reset" the device (which will wipe all you have on it) or have it looked at for 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.