Then you should use the original remote and select the player as source. Sounds to me as if he has selected the menu to disappear automatically, the screen visual to something like funnelweb (which will react to the source), and you have nothing connected to the input connector (the MIC input). Result: blank screen.

He's not damaged it. If you plug something into the harness connector I suspect that it will start showing activity on the screen. Is there a small dot right in the middle?

Change the source back to player.

PS. I would suggest when he comes close to your empeg in future, you remove one of his arms to indicate just how serious you are when you use the phrase "Leave it alone".