Could well be worth removing the cover and looking for a wire that isn't connected to the back of the power knob. If not trace the wires to make sure they are connected at the other end. Might well be a problem with the switch otherwise. These things are fairly simple inside.
Someone else is bound to be able to point you in the right direction if it's a common problem.
Ive had this problem before on a similar machine, a coil that goes through the switch in the Mains transformer was open circuit. Unfortunately a replacement transformer would cost more than a new machine..
Hope im wrong and its just a wire from your switch fell out.