Cell membranes rupture as water inside cells expands upon freezing.

The cascade of chemical events that constitutes cellular life stops.

Death follows.

This is why extremely cold temperatures reliably kill biological tissue.

This is why frostbitten tissue dies and must sometimes be amputated.

This is why the internal organs of animals that freeze solid do not recover when thawed.

The cellular damage is irreversible.

Jean Hilliard, by all visual and tactile evidence at the time of her discovery, had been frozen to the point where medical convention would have declared cellular damage irreversible.