Can you answer these questions, and I can help you out with what may have happened
What type of stubstrate do you have?
How many crabs?
What do you feed them?
Do you have a humidity gauge?
What type of tank do you have?
Do you use tap water, dechlorinated water, or salt water?
Do you supply then with fresh and salt water?
I've got 12 hermit crabs, and know almost everything you need to know about their care. Please answer those questions and I can try to help you figure out what happened, and how to prevent this from happening to your other crabs.
If you want your crabs to live long and healthy lives, you need at least a 10 gallon glass tank, with at least 3 inches of sand substrate. You needa humidity gauge and to keep the humidity at 76% with use of a spray bottle. You need to use DEchlorinated water, not tap, tap water kills. You should also supply then with salt and fresh water. Don't use store bought foods. They have harmful chemicals in them that are not safe for your crabs, you should feed a fresh diet with fruits and veggies.
If you have any questions, email me at Shelbyns@mail2me.com