Hammond Sk1 C key is dead and not producing sound

I have a Hammond Sk1 88 and the C key to the right of middle C stopped working. At first it would make a sound if you pushed the key down hard, but now nothing. Just a dead key. Could it be the contact strip? All the other keys work fine.

That is almost certainly a faulty contact strip. The replacement part is here:

It’s possible the contact board is damaged or dirty, but the rubber strip is a much more common problem - especially since the note worked before but only when pressed hard. And the strip is much cheaper to replace than the board!