They were at the party with Doctor Mixer, but they likely heard the news about the killings even if the hospital couldn't reach them via phone. Wouldn't they drive to the hospital once they found out Laurie was one of the attempted victims? Did Michael possibly kill them like in Rob Zombie's 2007 Halloween film?
I know this thread is 4-years-old, but I'm going to reply anyway. I don't know where they were, but it always seemed to me like they didn't care about Laurie very much. In the first movie, the way her father reminds her to put the key under the mat and then in the dream sequence in the 2nd one, the way her step-mother, as we find out, says, "How many times do I have to tell you? I'm NOT your mother." And then we see little Laurie visiting Michael all alone in his hospital room. Why would they let her go in alone? Yeah, I don't think they cared about her.
The father just made a simple request. I don't see how he was being nasty about it. And while the mother was a bit cold with her delivery she was merely telling Laurie the truth.
I think it's rather indicated Michael killed them offscreen. It makes sense given we don't see them by the end of the movie and the following sequels never had them seen or brought up again. H6 would have been the perfect time to bring them back and have them be the ones living in the Myers house, but we got extended members instead. I wish more care was put into their character statuses. They could have been the ones raising Jamie in H4.