Try turning it off restarting it in "Safe Mode" (instructions on how below). If it starts up then you probably have something (an app, software, etc.,) on the device that is messing with things. If that is the case, you would need to remove whatever is causing the problem, then restart the tablet out of "Safe Mode" to see if it is resolved.
If it won't start in "Safe Mode", you may have to try a "Factory Reset". This is not as severe as a "Hard Reset", which would delete everything and restore it back to its original setup. A "Factory Reset", however, will remove some personal data. So it is best saved for after having tried other options first. A "Hard Reset" is something to be used a a last resort.
To get into "Safe Mode" please do...