The potentiometer behind the knob MIGHT be the issue but that's unlikely. If it is, you can check by putting it somewhere in the middle and measuring the resistances between the three pins. They're a common off-the-shelf part.

More likely is that the temperature sensor is broken or disconnected; it will be a tiny bead on the end of some really thin wires, somewhere in the bath near the heater. On a CPP2 (with pump), the sensor is inside the pump housing.

It could also be the output transistor that switches the element has died shorted. Either way, you will need some electronic-debugging skills to find the fault. Schematics are readily available with some googling...