As there might be many possible reasons causing this issue and many possible solutions to this, we want to know more specifics.
What exact Mac and Mac OS X version do you have?
Where do you get no sound (speakers, headphones, line out)?
What specific steps have you already taken?
So what were you doing prior to it not working? Did you have some kind of external MIDI device plugged into it? This may help a bit or did you just turn it on today and no sound at all for no reason at all?
So what were you doing prior to it not working? Did you have some kind of external MIDI device plugged into it? This may help a bit or did you just turn it on today and no sound at all for no reason at all?