I know this problem from my own TW.
You will need a long pliers (telephone pliers) and a bit of raw force.
Grab the plunger and try to turn it a bit.
You can try to clean it with a some fine grinding paper. But don't remove to much material.
It can hold again on the cable, but I can not promise it.
In the most cases you have to buy a new plunger.
One problem: the TW125 is really expensive (~100 EUR). But you can use the SR125 plunger (~50 EUR).
For pictures you can use imageshack.us