During an emergency mission in space, the X-Men find their shuttle damaged. Jean pilots the shuttle back to Earth, but is exposed to fatal levels of radiation. Dying, but determined to save Cyclops and her friends, Jean calls out for help and is answered by the cosmic entity, the Phoenix Force. The Phoenix Force, the sum of all life in the universe, is moved by Jean's dedication and love and takes the form of a duplicate body to house Jean's psyche. In that instant, the Phoenix Force is overwhelmed and believes itself to be Jean Grey and places Jean's dying body in a healing cocoon. The ship crashes in Jamaica Bay, with the other X-Men unharmed. The Phoenix Force, as Jean Grey, emerges in a new green and gold costume and adopts the new codename "Phoenix", with immense cosmic powers. Meanwhile, the cocoon containing the real Jean Grey sinks to the bottom of the bay, unnoticed. Phoenix continues her life as Jean Grey with the other X-Men, joining them on missions and saving the universe. During the "The Dark Phoenix Saga", Phoenix becomes overwhelmed and corrupted by her first taste of evil and transforms into a force of total destruction, called "Dark Phoenix", consuming a star, inadvertently killing the inhabitants of the star's planetary system, and jeopardizing the entire universe. However, Jean's personality manages to take control and Phoenix commits suicide to ensure the universe's safety. (Wikipedia)
Thanks to the Avengers and Fantastic Four, Jean Grey came back to life just in time for the first issue of X-Factor in 1985.