Probably replacement of the antenna mast with the nylon part that extends the mast.

Power Antenna - Power but no movement
 

When I switch on the radio, the motor runs but does not extend the atenna.

Antena moves up and down by hand and does not feel connected to anything.

Just opened up the unit and the nylon coil is in fine condition but came out altogether when I pulled on it.

Where do I go from here ?

The cord has broken. Can I replace just the cord?

I'm only aware of the mast replacements, which include the nylon cord attached to the retracting mast. I suppose you could poke around a "breakers" lot and find a used one, but the replacement masts are quite reasonable.