After a long an exhausting walk a man falls down and says:
My leg! My leg's gone. He had a spasm?

He has lost all feeling in his leg - only one possibility!
When we lose temporary sensation in a limb (leg, foot, finger), we say that it is "asleep". One of my teachers several years ago got up from her seat after quite a time of having one leg crossed over the other; her foot had "gone to sleep", and she fell and broke her ankle because of it.
Teachers: We supply a list of EFL job vacancies
Thanks, opti!
 Philip's reply was promoted to an answer.
Site Hint: Check out our list of pronunciation videos.