No Sound

My iMac lost its sound today. It was working perfectly, then the next time I wanted to listen to something, nothing, though the alert sound works. Any ideas on what's gone wrong and how it can be fixed?

Many Thanks.

If the alert sound is working - then sound is working.

What are you listening to and in what program? I've found when I switched outputs (like from my headphones to my display Audio) some Apps do not like the switch. For example - if I switch outputs - Chrome won't output sound until I restart Chrome.

Secondly - did you plug anything into the audio jack? Of late I've had no problems plugging and unplugging headphones from the headphone jack - but I have had issues before where something didn't switch when I unplugged headphones.

Any more details - what you are using, what are your audio outputs, etc would help debug the issue.

Thank you, Ivan. Sorry, but I should have been more specific. I have tried ITunes and no problem, but with YouTube there is no sound when I unclick mute. There was no problem early today , then there was the problem.