recover is the small utility program that comes with NetHack. It is used to turn temporary level files into a save file, for example after a power outage during a game.
On Linux and other Unixen, you usually invoke recover like this:
recover -d playground 1031foo vultureseye-recover -d playground alock
where playground is the Playground path where the temporary level files were put during gameplay, and 1031foo is the file name of the temporary files, without extension. The number will most likely be different; it's the UID of the user running NetHack. foo is the player name used in NetHack. In Vulture's, the name is hard-coded to alock.
If you are using official Nethack and your distribution does not come with recover you can download it from the official download-page at http://www.nethack.org/.