when does elena turn her humanity back on

Elena turns her humanity back on in The Vampire Diaries in Season 4, Episode 21, after Damon and Matt orchestrate a plan to force her to feel again. Damon pretends to kill Matt, which triggers Elena's emotions and causes her to regain her humanity, making her feel guilt and other emotions once more. This is after she had turned off her humanity in Season 4, Episode 15, to cope with the immense grief of losing her brother Jeremy and the pain of her new vampire existence.