Some times the driver freaks and decideds there is nothing plugged into the hdmi port so it turns it off. I would power down unplug and repluge the hdmi then power back on. If that does not work I would use a vga cable and try again. I would then install the amd drivers from the amd site and not from the CD.
The fact that you lost sound may not mean anything because often the motherboard onboard sound driver is set as default sound device in windows.