I had this problem tonight as well after renting a movie and being unable to play it. Did some searching around, found this thread and another one which ended up solving my problem. Looks like it was an HDCP handshake error between the TV and the AppleTV. In case anyone runs into this issue, try this:
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).
Original thread/poster:
https://discussions.apple.com/thread/2794045?answerId=15021573022#15021573022
Once everything was powered back on the movie started to play right away. Hope this helps! And thanks to the original poster, "Ray"!