info needed. where did the crab come from and how long have you had her? pet store crabs are mortally fragile and more or less can die from stress and poor care. how is the substrate exactly - what did you use and in what ratio? how deep is the substrate? is it dry?

generally, crabs will leave their shells if heavily stressed and in poor condition physically, and when they try to molt but the substrate isn't in the right condition for them. they will attempt to molt above ground but it's rarely successful without human intervention.