He's not dead.
Michael gets up and starts to strangle Laurie. She is able to momentarily unmask him, and we see his face. Just as things look dire, Loomis busts in and repeatedly shoots Michael. He had seen the kids fleeing the house, and instinctively knew Michael must be near. Michael stumbles back over the railing on the second story and falls to the ground.
Between sobs, Laurie asks Loomis if it was the boogeyman, to which Loomis replies: "As a matter of fact, it was."
When Loomis goes to confirm where Michael has fallen, the body has already disappeared into the night.