Its a HDMI handshake problem. I had this months ago and found a cure.
I did these things 1. Unplug the hdmi cables. First tv then apple tv 2. Unplug the TV power and the AppleTV power, let them sit 30-60 seconds, 3. Plug in hdmi cables while power is off 4. plug the tv power in first, then the ATV power. This worked for me! (I was told it's an HDCP Handshake error, and the handshake is forced to be done again by unplugging power, but make sure you unplug both devices and plug them in the order above).
this solved my problem instantly!
I should of known this problem was down to hdmi and it's drm handshake nonsense
I can see exactly why steve jobs hasn't put bluray or hdmi on macs
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For me I just unpluged the hdmi from the tv while atv2 was running. Then pulled out power lead from atv 2. Plugged back in power lead and plugged hdmi back into tv
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At least my faith in apple has been restored as this hdmi drm business is entirely out of their hands. Apple products should just work