hermit crabs can start to hibernate below 68 degrees. However they aren't evolved for true hibernation. If it stays below 62 for at extended period they might not recover.

Why is my hermit crab not moving?

Notice lack of movement.

If your hermit crab isn't moving around much, the water in its habitat or even toxic paint on its shell could be killing it. Lethargy can also be a sign of stress. Recognize that most hermit crabs like to play and will move around their habitats a lot if they are not molting or sick.

Do hermit crabs leave their shell to die?

Without a shell, it leaves your hermit crab completely vulnerable to heat, light, and air. They can quickly die without it. It is common for crabs to leave their shell while molting. Once they shed their exoskeleton, they will re-shell themselves.