Have you tried several different HDMI inputs on the receiver?
If a different input works, then the original input might have gotten damaged.
Also, have you tried a different power-on order?
Often turning on TV first, then AVR, and finally player device (HTPC in your case) works best. Of course, you should wait until each finishes booting before turning on the next, so they're ready to transmit and forward the EDID info when asked by the PC.
After doing that power-up sequence, you might check to see if the PC detects the AVR. If it is detected, it should be listed in the "Playback devices" menu.