Tried rebooting your PC since it stopped working, have you got the latest HDMI drivers - tried replugging the HDmi cable in etc?
I get this on my media box from time to time. Go to your sound setup in control pannel, you'll probably find the default playback device has reverted back to your audio card, rather than the hdmi out
Which graphics card is this?
My Ati HD5xxx started having problems with HDMI audio when I installed the 11.9 drivers
I updated my HDMI Ati Audio driver from the realtek website and it fixed the issue