@naf26. The contacts under the buttons are a kind of electrically conductive rubber. When this is pressed it touches two terminals of a conductive ink that have been printed on the board.
This allows a small amount of current to flow across these terminals which tells the casio that a key has been pressed. If the ink has been removed or damaged you can fix it with a conductive ink pen. Unfortunately these pens are very <a href="http://www.mouser.com/ProductDetail/MG-Chemicals/8420-P/?qs=sGAEpiMZZMvcAs5GUBtMdfkI%2fJokWwe5" rel="nofollow">expensive</a>. I found one that is cheaper from <a href="https://www.sparkfun.com/products/11521" rel="nofollow">Spark Fun</a>. It looks like it should work but I cant vouch for its quality.