I would guess that the hard drive platters seized, and the cooling made it shrink enough to un-seize.
By freezing the drive, the metal would shrink just enough to unstick the head. Of course the location where the head crash had happened would be corrupt, but the rest of the data would be fine (mostly).
I spent 40 minutes copying that drive, got every bit back.
Boss asks, “What are you doing?”
“I’m milking the data out of this drive!” ;)