It's hard to say, but since you turned off/on power to the AVR, it may be the culprit. But also, if you have a 12 trigger plugged into the XPA, it probably reset that too. Were the lights orange on it?
Your AVR is like a computer, so you have basically reset it when cycling the power. Did it seem warmer than normal? My Denon 4810 runs pretty hot so I keep all the vents clear of obstruction.
If it does it again do this, cycle power on the Emo first, if still no audio then stop the Bluray and restart the movie. Still no sound, cycle the power on the Oppo. Still no audio, switch to a different audio source on the AVR. If you get audio, switch back to the Oppo input. Still no audio there, try reinserting the audio cable. If you get audio, it may be a bad cable. What are you using for audio cable wise? HDMI, Optical, Coax...? Is tv sourced from the AVR for video or straight from the Oppo? You'll have to do a little investigating next time it does it narrow down the culprit.