Fiddler crab with no access to land in completely fresh water?

How long will the poor thing live? She's been fine up until now. Right after my daughter got her at Walmart she was very active and seemed very healthy. Now she barely moves and hides in corners and stuff. At Walmart they claimed the crab needed no special care. We now know that's different, but what do we do? She's acting very sickly and I don't know if it's because of the lack of salt and air or if she's just ill. .