If the tablet is under warranty, I would contact your service provider or the manufacturer to have the device repaired or replaced. Why? Wel doing many things to resolve this may well void the warranty, so it is best to go through them if you can.
Now if you are out of warranty, the first thing I would suggest you try is to locate the "Reset Button" on the device. It should be a little pinhole either on the edge or where anything plugs in (cord, card, etc.,) to the device. Using something small (like a pen tip) depress the "Reset Button" to do a "Factory Reset". Do not use anything sharp (like a pin) as you can damage the device. Note: You 'may' lose some data on the device when doing this.
You can also just let the device run down until it is totally out of juice. Which would turn it fully off. Then charge it and once charged, restart it.
I would suggest, in either case, you restart in "Safe Mode" as you may have installed something malicious on the device without realizing it and that could be causing your problems.
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 safe mode, simply repeat the steps above, and you should see "Safe Mode: Off" rather than On.