I've just updated to the latest version on Apple TV and it won't work

All I see is a constant white flashing light on the main apple box and then on the tv screen it shows a picture of a cable (plug) and above it an iTunes music symbol. I've checked all the cables are plugged in correctly, I've turned off the tv, rebooted the router, unplugged and left for 5 mins then re plugged and turned back on and I still have the same problem! Same picture which looks like it's telling me I need to plug it on again etc etc and same flashing light.

Can anyone help me?