Basically “power on” with also “volume up” and home button pressed. That will lead you to the “Android Recovery Screen”. You will want to restore the factory settings.
I stopped using the tablet myself quite some time ago, I prefer my MacBook Air. I let my 10 year old son use it exclusively. I guess one of his latest apps causes the problem. Of course I would like to be able to remove the latest apps – but in state the tablet is currently in, there is no way to really do anything but wait for the instant reboot.
My poor little son will loose all his game states i.e. progress. Actually his biggest fear was, that all the purchased games would no longer be available. But that’s not right. We can determine what to re-install from within Google Play.
Update 2017-03-07: Recovery mode: Clear Cache: Clear data.
Update 2017-03-12: Our tablet has no home button 😕 So we cannot apply this recipe.