Try starting it up in "Safe Mode" and see if it will fully turn on. If it does, your access to things will be limited, however then you know the problem is software. So virus, malware, bad app, etc., which you need to remove. If, however, it won't load even in "Safe Mode" it may require a "Hard Reset" of the device, which would wipe everything on it, or possibly even be a hardware problem.
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.