As I recollect, Prime was never supposed to really come back. He was brought back to life temporarily by the Quintessons as a tool of destruction, in what was supposed to be the grand finale of Transformers, in the episode, 'Dark Awakening'. One of the best episodes ever, IMO. It should have been left as the finale, as it was a superb ending. However, the Transformers were renewed the next season, and they saved Prime from his doom (that was really hard to swallow considering that Prime was flying into the explosion and coming to pieces in his last scene in Dark Awakening), where he re-appeared in the 2 part 'Return of Optimus Prime'. If I remember correctly, they even modified the Dark Awakening episode to show the next season right before the 'Return of Optimus Prime' in order to show him getting saved by some humans in a space ship.