If we're talking about the foot control with the electronics and the potentiometer on the bottom, there's a capacitor that needs to be replaced. We used to get them at Radio Shack, but they closed. When the capacitor goes out, the foot will stutter badly.
If the black cover is broken, that's a fire hazard, and the whole thing should just be replaced. We just use the cheap electronic foot controls, and attach them to the existing cord. It works pretty good, and costs about $40.
If there's no power going to the machine, it could be a break in the cord instead of the foot control.