There have been loads of posts on AB from people who've reported losing sound on their computers when, in fact, they only mean on a specific website (or, perhaps, only when playing CDs but not when listening to online content).
So, before proceeding further, go to a Youtube video and see if you can hear the audio from it. Then go to a video on, say, a BBC page and see if you can hear the audio there.
If you can hear the audio on a BBC page, but not on Youtube, you've clicked on the 'Mute' button on a Youtube video (probably accidentally) and that setting has been remembered for all other Youtube content. (It's a problem which has been reported countless times here on AB!). The solution is to go to any Youtube video, place your cursor over the bottom of the video and look for this button:
http://www.upl.co/uploads/Muted1566769610.jpg
Click on it to turn the audio back on.
If your problem is more general though (i.e. affecting
all audio content, and not just that from Youtube) try clicking on the loudspeaker symbol at the bottom right of your screen and then on the Mute/Un-mute button below the volume slider. If there was a little red 'No entry' symbol there before you started, that should fix the problem. If, however clicking down there has actually made the 'No entry' symbol appear, clicking again might then restore your sound. (Also, while you're there, check that the volume slider isn't right at, or very near to, the bottom of its range).
If you then still have problems, please post again, Henry.