Well, there's two possible explanations. If you go on your settings on your xbox using your old lead, there should be a setting under display I think which turns your xbox into HDMI display. Because it will have been switched, the display on your TV should be then be blank, but when you plug in your HDMI lead it should (in theory) be working. Either that, or your HDMI cable has broken. Personally, I never had to do the first option, because I just plugged in my HDMI and away I went.