Whats wrong with my hermit crab?

Ok so my hermit crab ,Zig, sits inside his hut all day and night. He has been like this since Sat. May 21st. That is when his sister ,Zag, died. Then on Sun May 22nd I went to Petco to get another hermit crab whom I named Honey Bee. Honey Bee is fine. Yet it is now June 15th and Zig still sits in his hut never comes out to eat or drink. I know he is alive because I check on him every day by picking him up and playing with him. I currently have him seperated inside an I.S.O tank until he gets better. What is wrong with Zig?