Xbox 360 won't display HD :(

[Big G]

I setup my xbox on Monday using the composite cables as I didn't have a HDMI cable at the time so now i have PSone quality graphics, but at least it's playable.

I borrowed a HDMI cable last night to try, but the TV just says no signal. When I goto the display settings the HDTV tab is greyed out. The TV is a cheap 20", but it does have a HDMI plug on it (only 1 lol).

I've used the HDMI port before so I can confirm that works and I've been told the cable is ok... I've got my own cable+xbox AV output thingy so I can connect my turtle beach properly which will hopefully be here today.

My next plan was to set it back to factory and start it up with HDMI plugged in.