One of the keys on the synth is losing the sound. Could it be a simple contact that needs cleaning and if so is it difficult to clean. I have no manual

I have the same problem with some keys, and i simple clean it and now is ok! It´s a simple rubber conductive, simple clean it!

It is most likely just dirt or something in between the rubber strips which are under the keyboard, something like one per octave, and the contact point under the rubber strips. You have to remove several keys to remove the rubber strip, then the contact points will be exposed so you can wipe or blow away dust, even the smallest particle will interfere with the sound.