Have you tried starting up the tablet in "Safe Mode"? If not, I would see if it will do that. Should it work, then you know to locate any recently installed apps and remove them, and to check for viruses, malware etc.
Should it not load in "Safe Mode", then you may have to "Hard Reset" the device. Just note that will remove all personal data from it.
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...