Help me understand and fix this dimmer circuit
The pot resistance and the cap form an R-C charging ramp voltage.  When this voltage, divided by 83% (1K and 5K voltage divider) exceeds the SCR trigger voltage, the SCR conducts.  Thus, for each half-cycle of the power line, the light cones on some time delay after the zero crossing.  As the pot resistance gets smaller, the ramp time shortens, and the light comes on earlier in the half-cycle.

From your symptom history it sounds like the pot wiper was not making good contact with the resistance element, eventually not making contact at all, but still was making contact with the element end point where it is attached to the pin.  Now, not even that is making contact with the wiper.