Loomis knew better. He yelled out "…he’s still breathing."
Myers was just momentarily incapacitated, and since it was Halloween, he was just playing a play dead trick. This was Michael’s game.
Michael died at the end of the movie, as was originally intended.
Norman! What did you put in my tea?

Before there was a Halloween 4, yes. Once they decided to make a part 4 (with Michaal), he did not (die).
Michael is a supernatural entity. There are limits obviously, but he's not a regular mortal man. He doesn't heal as fast as say… Wolverine, but there's clearly some degree of regeneration going on. A coma makes sense.
I imagine sometime sometime after the events of part II he showed signs of life. Maybe in the morgue, someone noticed he's breathing. Not moving, but showing faint signs of life. And he remained in a coma until his body healed enough so that he could rise again.