Why is there no sound or image?

@jean19230 "The only way I can turn it off is to unplug it" and the no sound or image, sounds very much like a main board issue. I would suggest that you remove the power cord, then remove the back cover. Now take a look at the boards. Check for any odd-looking component. Take some well-focused pictures of your boards and post those with your Question. Once we can see what you see, we can guide you a bit further on how to test the board. For this you will need a multimeter. Nothing fancy, it's just to test a few DC voltages.

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