My Macbook's headphone jack stopped working, how do I fix it?
(posted as an answer so you can accept it and mark this issue resolved)

Have you checked the Sound pref pane in System Preferences? There's a tab labeled "Output" where (I'd expect, as I don't have a portable in front of me just now) you can switch output between built-in speakers and the headphone jack. If the first toggle doesn't work, toggle it again.