Why has my hermit crab buried himself??!!

Australia
January 31, 2007 7:15pm CST
My boyfriend and i boughthermit crabs a couple months ago. We bought 7, but one of them , Moggil, decided to bury himself about 4 weeks ago. We had seen absolutely no sign of him until a few days ago when i found his shell, empty. I originally thought that he was just moulting-because I know crabs bury themselves for a week or two while they moult, but ti's been far too long. There is no smell coming from the tank- so i'm not sure if he's dead. Does anyone know what could've happened to my poor Moggil?