I had a similar thing happen to me just a few days back and nothing would make mine reboot or start for a while, NOTHING.. In the end I found what I thought was one of those "reset" holes in the side of the tablet and stuck a tiny screwdriver down inside it. I am sure I felt something go "bump" inside there as I prodded around although I can't be sure as I was kind of annoyed a bit, however a few seconds later it let me power up again. I simply held the power button down for about 10 or 15 seconds and let go and it restarted. I wasn't downloading at the time but a similar message popped up on the screen ("not to turn off" while it saved my work), after I quit out of a program, and the screen went black after a few seconds of freezing and it seemed as if it was dead for a few minutes, but it did not power down, it just stopped like someone ripped out the battery. It is going again and works well...
I wish the best for you...
Android Al...
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Edit a bit later... just to let you know that I have only been using a tablet for about one week and a few days, so I am very new to them... Cheers... Al