John Winchester Dies To Save Dean\n\n Supernatural season 2, episode 1, "In My Time of Dying," opened with Dean in a coma. Desperate to save his son's life, John summoned Azazel and made a deal with the demon  he sacrificed his life and soul so Dean would live.

In order to force Mary Winchester into making a deal with him, Azazel snapped John's neck while possessing Samuel Campbell. As part of his deal with Mary, Azazel resurrected John and erased John's memory of his death.