It sounds like you (or someone) enabled the Internal Mute or "External Speakers" option in the menu. Have you checked to see if that's the case?
What model do you have?
My television has always worked and connections have not been changed. All of a sudden I get no sound. When I try to turn the television up it reads "not available." There is a digital cable box connected to the television, but it has always worked. I attempted to play a dvd, and there is still no sound. Everything else is fine, but it shows not available, I don't understand why it worked that morning and when I came home after being at work all day it no longer works.