Try and see if it will start up in "Safe Mode". If it does, then you probably have something on the device (virus, malware, etc.) that is the cause. You would need to remove that to get it to work correctly. If that doesn't work, you may have to do a "Hard Reset" of the device. But I would save that for a last resort, as it will wipe all your personal data.
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.