For windows go to the start menu and type run (or click on it if you have xp/vista). Then type %appdata% and enter your minecraft folder. Enter your saves folder and copy the map into there.
Have you checked your .minecraft/minecraft folder I'v found a couple of times if it was a recently deleted save I could still find it there (I have no idea why) anyway if not I would just do what the guys and gals above me said..