I don't know anything about these crabs or much about crabs in general, but it he was a land species with lungs rather than gills it is possible that he drown while molting in the water source. That is if it was deep enough. Could have been from complications while molting too. if he passed I'm sorry for your loss.

Purple Vampire Crabs live on land and in water i have had it for over 2 weeks it was doing fine yesterday.I figured he passed away from complications from its molting. It sort of makes me mad and sad at same time because i just spend hundreds of dollars to make her a habitat that resembles there natural habitat. I havent put her in it yet because it has to run for 3-4 weeks. DANG IT. Thank you for responding.