my xbox 360 runs,plays audio,but no video shows..i tried replacing cables but it did not help
Make sure your Xbox has the correct video settings. If it is set to 1080p and your TV only supports 1080i then no video will show. Also make sure the switch on the cables are set to TV or HDTV depending on what your using. TV setting should work on a HDTV if you want to make the edits to your video settings.
Also make sure your TVs connection is working properly, with other devices. Also make sure to test the Xbox with other games.
If neither of these work then the above poster is most likely correct and the GPU is dead.