I had the same thing happen: Casio Previa Px-160 got bumped while moving and key popped up as in the original poster's picture.
I was able to fix this by disassembling pretty much the whole keyboard to get enough clearance to loop the key back over the hammer. The screws are all pretty easy to find : you need to remove all along the back, sides, and the internal ones inside the 8 access hatches on back. There are a few remaining screws holding the whole key assembly. After removing all of those plus the retaining screws for the keys you need to fix you can lift the whole keyboard out enough to slide the key back in place.
It's a huge pain but fairly easy.
