iMac headphones no longer detected
I have a late 2009 iMac running OSX 10.8.2.
I have used my headphones successfully many times but today when I plug them in the internal speakers stay on and the headphones do not work. Also, in system perferences sound panel with Output selected, Headphones does not appear as an option.
I have just restarted the computer and the result is the same. It seems to me that even if the jack is damaged that Headphones should still appear in the Output panel so I believe that this is some kind of software problem.