Within the phono jack is a set of contacts and a switch. It sounds like it has gotten damaged.
You'll need a flashlight and a toothpick see if you can look down into the outlets hole compare it to the another outlet from a second system.
If you don't have a second system to compare it to you may want to visit an Apple Store.
Sometimes you can fix the contact otherwise you'll need to replace the outlet which is a very big job! As the whole logic board needs to be taken out to either to be swapped out or to gain access to the connector block. Here's an image were you can see the block (the left most metal can) logic board connectors.
You may want to try using a USB to analog adapter if you really need the signal output.