Hi.
You need a new potentiometer.
As for what happened.
Well...
Some of You may remember certain someone telling in every knob replacing thread to break the knob if it won't come off easily.
Others insist on prying it with all kinds of things that can be used as levers.
In theory, You may be able to repair the potentiometer, but unless it's something really, really special, don't bother.
On that pic above, the tangs You can see hugging the top plate and the GF plate (with the resistive) track, can be bent, and the cup comes off.
The wiper (the piece that rattles inside the now defunct potentiometer) is usually riveted to the shaft that You have permanently attached to the knob ATM. When You eye the way it was attached, it soon becomes very clear why the repair is neither feasible, nor long lasting.
When I was a kid I did repair plenty of potentiometers. The really old ones had a marginally better success rate than the CP (even back then) potentiometers.
Regards
Sam
