Okay, I am having a very strange problem that is pissing me off to no end.

I run my Xbox 360 through a Pioneer VSX-819H A/V receiver to an Insignia 42 inch 720p HDTV. It's simple to hook up —*HDMI from Xbox to receiver, and HDMI from receiver to TV. Everything worked great, and I got my surround sound. Three days ago, I removed the HDMI cable connecting my Xbox to the receiver to use at a friend's house. When I brought my cable back and plugged it back in — exactly the same way as before, Xbox to receiver — I now get no sound from my Xbox. What is going on? I've google for solutions but have not found anything that works. The cable is not at fault; I have switched the two and still get no audio. Meanwhile, when I watch regular TV, the audio still works. How can I fix this? I can't play most of my games without sound!