It is the Q key that fell out. I have 2 pieces 3 including the key its self.
How you would replace it depends completely on...
Now... here is what I mean.
If you just caught your finger under the key cap (IE, the square piece of plastic that has the Q printed on it) and accidentally popped it off... and none of the tabs on the underside of the cap are damaged, and the two other pieces are merely the two halves of the X-hinge that keep the key cap stable when perched over the rubber nipple/plunger that actually performs the keystroke... then it would simply be a matter of re-assembling the two haves of the X-hinge together, connecting the key cap to one side of the x-hinge, mounting the x-hinge to the aluminum clips on the keyboard, and lining up the other half of the x-hinge below the key cap and above the other clips on the keyboard... then, with a downward press on the key cap, everything would be snapped into place.
However, if the clips are broken on the underside of the key cap, or there are any parts of the x-hinge broken so it doesn't go together properly, or the pins at the corners of the x-hinge are broken so as not to attach to the key cap or keyboard, or the aluminum tabs on the keyboard are broken... then it's not going to go together well.
If that didn't necessarily make sense... or if you'd like to read it described with smaller paragraphs or different terminology, there is this guide here at Laptopkey.com