Unfortunately I experience the same problems. I leave my PC (Intel NUC) on for 24/7, because I also use it to record live-TV. After the upgrade to LibreElec 8.0 (coming from 7.03) I have this problems. If I don't use the PC for a couple of hours then I have sound, I can start music or even a tv-show, only sound, no video. Turning on & off the TV, receiver doesn't make a difference. Switching to another HDMI on TV or receiver and then back to the correct HDMI also doesn't help.
What's different though: I don't use CEC. I just use my Logitech Harmony to control my devices.