Putting it back on wasn't hard, it just took a little while to figure out.
After the key came off, I had a hole in the keyboard that looked like this.
And the key and a U-shaped pin that came flying off when the key went.
The underside of the key has two tiny clamps at the top: one for holding the pin and one for attaching the key to a bracket on the keyboard. The top set of brackets is for the pin.
The pin just snaps into the upper set of clamps.
On the keyboard, theres are two upside-down-U-shaped brackets that you have to slide the legs of the pin into.
There's also a plastic bracket that I've lifted up with a screwdriver here. It has two pins sticking out from the top that snap into the bracket clamps on the key.
You have to slide the legs of the pin into the brackets on the side, then snap the bracket clamps on the key onto the pins sticking out from the bracket.
I slid the pin into the holes, then slid the top of the key down into the hole in the keyboard so that the bracket clamps were just behind the pins on the bracket. Then I used a small screwdriver to push the pins into the clamps until they clicked. It's a very quiet click.
The picture shows how I had it positioned. Didn't have enough hands to snap a picture while using the screwdriver to push the bracket pins into the clamps. It's pretty straightforward though. Just move the key and the bracket so the pins are right between the arms of the clamp, then push the pin in.
After that, just lower the bottom of the key and push down so that the bottom snaps into place. Should be good until the next time you drop your computer.