This happened to me with my Vizio soundbar. I had an app up last night and the LG went to a screensaver and then I turned it off. When I turned it on today, no sound from was working along with a cancel sign every time I tried to change the volume. Nothing in this thread worked (didn't try resetting either TV or soundbar).
I fixed it by first unplugging the soundbar power cable then plugged it back in. Then I unplugged the HDMI Arc cable from the TV. Then I went to to All Settings > Connection > HDMI Device Settings and turned Simplink off. Then I went to Connection > Device Connector > Soundbar and selected HDMI 2(ARC). Then I plugged the HDMI Arc cable back in to the TV. The sound test option was greyed out until these steps. After plugging it back in, Simplink was reactivated, I could play the sound test, and the soundbar was working normally again.
