I have been researching this problem for the past hour or so with no success. Basically I have been using my xbox on my tv just fine, but now I have tried hooking it up to my brother's tv and it just won't show.
He has Sky hooked up via to his tv with the very same HDMI cable and it works perfect, he is also able to connect anything else using the same HD port and cable, but no picture at all with the xbox. After trying all the obvious things I thought it might be because the xbox was using a different resolution on my tv, so I hooked it back up to my tv and changed the settings, but it didn't help (I tried all 5 or 6 different setting).
I also tried holding Y + RT when you start the console but I assume you need at least a picture to do that, either way it didn't work (tried it multiple times).
To avoid unnecessary suggestions I have already tried the following;
Checked tv setting and tested some variations.
Made sure the connection to tv and xbox ports is secure.
Tried the HDMI cable with something else to make sure it works/using a different HDMI cable.
Tried the xbox on a different tv to make sure it works.
Tried a different HD port on my tv.
Changed the resolution setting on the xbox itself.
Did the Y+RT thing on startup (with disc removed).
What could possibly be the problem?? The tv works perfectly with everything else but the xbox. The xbox works perfectly on otherTVs. The HDMI cable works perfectly on anything else.