As a studio tech, I'm often asked to fix any item that might come through the studio doors. It becomes a wide variety of repairs. Today I was tasked to fix a Nord Stage 3 Piano Keyboard key. I also made a detailed video of the process, so check that out for a true step-by-step.
I begin by checking the keys and noting which ones are not working. Then I remove pretty much all the screws holding the shell together, which are bottom sides and back(none from the top).
Once you have cleaned as many keys as you need to (or care to), reinstall the pads onto the PCB.
Clean up as much as you like, and reinstall the board onto the key bottoms.
Reinstall your ribbon cables and put the clam shell together. Don't forget the small machine screw which holds the internal brace.