Try starting it up in "Safe Mode" and see if it will load. If it does, then you have something on the tablet that is the problem. Virus, malware, bad app, which you will have to remove to get it to run correctly.
If, however, it won't load up even in "Safe Mode" then you may well have to either "Hard Reset" it, which will wipe everything on it, or have it looked at by a tech.
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.