Judging by the difficulty that you're expressing, I'm guessing it's one of the scissor-type buttons. Those are a bitch, but it is actually possible to get them back on, you just have to be very gentle because the plastic hinges are fairly easy to break (and if you are rough with it, the mechanism will come apart while you're trying to put it back in). I haven't done it in awhile, but if I remember correctly, there are the two pieces that fit into the bottomside of the button, and then two pieces that fit into the little metal hooks on the topside of the keyboard metal.
I believe it's easier if you try to fit the pieces around the hooks on the bottom, and try to flex the plastic properly to get them to fit into the little clips on the button itself.
Hope this helps a bit. If nothing else, just sit there for awhile, and examine it; that's how i finally figured out the proper way to get the key back on.