In 2018 it was announced that a multicellular organism - a species of nematode worm - had been successfully revived after having been frozen in the Arctic permafrost for around 30,000 years.
See: Viable Nematodes from Late Pleistocene Permafrost of the Kolyma River Lowland , Doklady Biological Sciences 480(1):100-102
It's not known how the organisms can survive for so long without any apparent damage to any of their systems.
The majority of complex animals sustain permanent cellular damage when frozen, even for short periods - primarily due to the formation of ice crystals within cells and tissues.
Note: Some fish are adapted to long periods of very low temperatures (−3.5 °C) due to the precence of biological 'antifreeze' chemicals in their system. See Wikipedia
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