Did you set it to Wumbo? Lolol.. yeah idk whats wrong DB. I've never had that problem before. Did you try a diff HDMI cable? It might not work too well on 360 unlike your PS3 :/
Actually It even stopped working for my PS3 as well. Same effect, sound in the background but all I had to do was restart my PS3 and it showed. I restarted the 360 and still no display. I haven't tried swapping HDMI cables yet. I'll do that now.
Alright tried swapping HDMI cables and still didn't work. Youtube method didn't work. It shows on my capture device that it's working fine but doesn't show on the screen at all. I'm starting to think it's the console.