Dave Stewart wrote:
I bought a pile of live rock that had been left outside. Temperature below
freezing.

Is it likely that all critters are dead?

Well, last month I did an emergency rescue of a salt water tank where
the live rock sat in an open carport for about an hour, and then rode in
the back of my car for another 45 minutes before being plopped into cool
sal****er at my house. I was sure that everything was dead, but all of
the rocks are now showing some signs of life.

Admittedly, the temperature wasn't quite low enough to freeze water, but
I'd say that in your case everything depends on how long the rocks were
exposed to the cold, and how sheltered they were from the wind. I'd
drop them in warm water for a couple of weeks and see if anything revives.