On a 21 mile bike ride yesterday, my front DR suddenly stopped working. It was dark so I couldn't tell what happened and kept on riding the remaining 11 miles.

Once I got it home and took a look at it in the light, it was apparent that the shift cable was very slack. I checked it from the brifter to the DR and it appears to be intact and attached at both ends. I pulled on it from either direction and it doesn't seem to be broken.

I can't figure out what happened. Any ideas?