Riddle me this:

My front derailleur won't move. I've tested it by pushing the mechanism with my finger and it appears to move the way it is supposed to move. But the cable won't move it. The cable is as tight as I can make it, but it still won't budge the derailleur when I move the shifter. Any ideas on what I can do to make it work?

To be up-front, while I'm comfortable many aspects of bicycle mechanicing, I've never once in twenty years felt like I understood how the front derailleur actually works and have always given up and taken it to my lbs after I've changed out the cable and messed with it myself for awhile.

I have a very old set-up. Non-indexed stem-mounted friction shifters. This bike was a 10-speed, but has been upgraded to a 12-speed. The shifter levers themselves seem to work fine.

The cable housing actually needs replacing (it's pretty shredded, but the cable itself is not broken).

I picked my bike up after some work not related to the front derailleur. My bike was in exactly the same condition as it is now, and it worked fine. Shifted through all the gears.

When I got it home, I don't know what I did, but I heard/felt something pop after I shifted into high gear, and now the cable won't move the derailleur even though the cable is not broken, and the derailleur appears to be working properly otherwise.

Any ideas on what could be going on?

I won't have a chance to go to the lbs during business hours for a few days, but I will be riding each day until then, so I would like to fix this myself if I can.