when I connect my 360 with AV cables I do have a picture but using an HDMI cable it doesn't show I even tried different ports and changed cables but I always get the same results, Please help

I'm currently having problems connecting my xbox360 to my TV using HDMI. When I connect my 360 with AV cables, I do have a picture, but using an HDMI cable doesn't work. I have tried different ports and changed cables but I always get the same results; it's not working.

Could someone help me figure out what the problem is?