One of my 3 hermit crabs just died.

I don't know exactly when she died but she has been not doing well for quite some time. She has been looking very gray for months. Several months ago she went down (presumably to molt) then went back down after just a couple of weeks and I didn't see her for almost 4 months. She has been up for I'd say a month or so and she wasn't eating. She was very pale, pretty much grey, not eating, not moving much, and spending most time in dark corners or next to drift wood. I have not been able to change the substrate/revamp the tank in a long time because for some context I'm 14 and I can't do all of it by myself (moving the tank and bringing sand up) so my dad helps me. And he hasn't been doing so and also the crabs keep going in waves of molting so whenever we are going to do it, one molts and we have to wait longer. So idk if she was depressed from the lack of revamp? Or what. My other two crabs have been doing really well, fairly active, eating, sleeping, spending time together. So idk what happened. She went down into a small hole that we have underneath a hollow log and she was there for several days. We were checking up on her but she wouldn't move. We tried feeding her but she wouldn't eat. I checked up on her again yesterday just to see she was dead. I could tell bc she wasn't moving at all and her eyes were together. I'm really upset right now and I don't know what to do. I'm planning on doing the tank later today/tomorrow. Also I do know she had problems finding shells but we had shells which appeared to fit her and the other crabs which are similar sizes. U think anyone could maybe tell me why exactly she died?