Prior to the advent of modern medical science, it had been observed that survival from severe hypothermia was possible, if uncommon. Many animals enter states of dormancy to conserve water and energy during particularly adverse climatic conditions e.g., dormice and bears, lungfish and some desert toads on Terra. However, there is a limit to how long dormancy can be maintained.
Metabolism continues, albeit at a greatly slowed rate.
Long term tissue preservation requires metabolic arrest. To effect this, cell water must be locked in place to prevent cellular proteins from working. It had been observed that rapid freezing limited the size of ice crystals formed. By Tech Level 8, it had been shown that animals such as frogs and mice could be snap-frozen, thawed and revived. Survival rates were predictably low.