the common thing that i have seen is, especially with units that people are heavy headphone users is the jack gets stuck on output for the head phones. if it happens again take a look at the jack to see if there is a red light, if so thats why there is no sound.
did read sometimes that you can use a toothpick or something similar to nudge the contact back to say it not on external. other wise have to replace the logicboard(motherboard,systemboard which ever term you wish)