The volume control on my Mac just went inactive -- a 'no entry' icon appeared when I tried to adjust the volume using the keyboard, and the visual control itself was greyed out.

Turns out the problem was that the jack on the lead running from my audio output to my stereo was half-in and half-out. The audio-out channel had been deactivated, but the built-in speakers had not been re-activated, so there was no sound channel available. It was most puzzling for a few minutes, until I realised...