Dead keys are quite common for electronic keyboards, especially analog synthesisers, to occasionally develop keys that do not produce sound. They are mainly due to dirt and other debris disrupting the signal that creates each key’s designated tone; cleaning these contacts will solve the problem. Understanding how to maintain two distinctive but very common designs, the Casio SK-1 “button contact” design and the Moog Satellite “rail contact” design, will solve the problem in most cases.
Remove the main key assembly from the keyboard's body enclosure. Again, take pictures with a digital camera so you can see how things are assembled. Be very gentle when removing the keyboard assembly because it will probably be connected to various circuit boards by delicate wire.
Remove the rubber strip of buttons from the circuit board. The rubber strip is not glued down but simply pressed into the circuit board via a few rubber nubs and holes in the circuit board. It will take minimal effort to remove the rubber strip of buttons from the circuit board, which should reveal a long strip of black contacts underneath.
Locate the "rail contacts" of your keyboard. The rail is a long, thin wire that stretches across most of the length of the keyboard directly underneath the keys. The rail is how the circuit is completed and the tone is accomplished by the small wires attached to each key that make contact with the rail when they are pressed.