Oh wow I have also had a problem, I ordered some 3/4 inch shells for the smaller crabs to get them out of the painted ones and they gave me 1 inch shells
I have a hermit crab named Spike and he did the same thing when he was small. He came back up fine but I honestly thought he was dead. If it were me, I would leave him. If he doesn't come back up soon I would do what KellyCrabbieLove said.
I have literally bought 2 dead hermit crabs before. When I returned one once the worker pulled him out of its shell If the hermit crab you have smells fishy or if it's limp he will be dead..