I have a weird question about the crabs in my fish tank?

so i have this big fish tank and i bough a male crab for it last week. then this last weekend i bought another male crab and also a female. well when i put the new crab in the tank it kinda looked like the 2 males were going to fight but i wasnt to worried, i figured they just needed some getting use to, so i went to bed. then this morning when i woke up the new crab was looking fine but when i came home for lunch he was upside down in the rocks so i took my net and sure enough he was stiff as a rock but i just left him in there and figured i would take him out when i got of work later. so 5 hours pass and i got home and there was no dead crab to be found! i cleaned the whole tank and couldnt find him! what could of happened to him? did the other crab eat him? (but it was only a few hours) or did he likesome howe dig himself in the rocks and fully die under there or what... i dont even know what to think any more... opinions would be great! i just want to find the little guy :)