Yamaha PSR-2000 sunken key

I have one sunken key on this PSR 2000. I removed all the back screws and the  entire keyboard from the  workstation and then removed the 21 screws holding the keys to the long strip and the 2 boards with 15 clips that allow access to the  rubber contact strips. Now I cannot figure out how to remove the keys. Each octave is made up of  2 sets of white keys (cegb) and (dfa) and one set of 5 black keys. They are nested into each other nut I cannot figure out how o remove them Can anyone help. Thanks so much.  Tom  What is the cause of a sunken key - i.e. - will not return fully to the up position - its about 1/8 of an inch below the other keys. Thanks