fg is a generic Linux/Unix utility, that resumes execution of a suspended process.
Sometimes, unwary users press "ctrl-Z" by accident. This suspends the game and puts it in the backgroud. With the message:
"+ Stopped nethack"
Attempting to restart the game by rerunning "nethack" will give the message:
"There is already a game in progress under your name. Destroy old game? [yn]"
Say no to the above. You just need to run the command "fg" in the original terminal to resume the frozen game.
If you have already killed the original terminal, then try killing nethack with "killall nethack". Then you might need to actually use the recover utility.