What key? The one you keep plugged - the emergency stop one? If so, I'm sure it's just a sensor that has to be triggered. I bet any old piece of plastic (that fits) would work. Do you have anything that size? Maybe rig something up with a plastic knife?
try calling the manufacturer. i am sure they have some kind of system for replacing them. i am sure people lose them all the time. especially when they have kids. my daughter has lost the knobs off of our stove twice, so after the second time, i asked the manufacturer to send us three sets, just in case.