Had a similar problem with a new unit that my son put in but on the lead between the xbox and t.v a slide switch had accidentally been moved over to HDTV while it was not connected to a high definition t.v. and switching it back solved it. If still a problem call the helpline in the booklet.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix games consoles.