HCA: Hermit Crab Association

One of my crabs shed his shell and is gone. I guess that he's molting, but wasn't sure if it's usual for them to leave their shell on the surface. He's never done that before. Is this normal?

They go down with their shells, are you sure it didn't change shells before it went down? I sure hope it did.

If you think he really is "naked" down there, then you might want to consider digging him up. It's never good for a crab to go naked, the only time a healthy crab will leave it's shell is when it's about to go into another shell, and that will be for a matter of seconds.
This might help a bit --->
viewtopic.php?f=27&t=38729
Crabs go naked for a number of reasons, if he is a new crab, he could be suffering from PPS
If you find him naked, the link will have instructions, but I can give you more help if you need it just keep in mind how very delicate their abdomens are, they're like thin skinned organ filled balloons.

He's naked. I'm fairly inexperienced at owning hermit crabs. I just learned that I had the wrong kind of shells for them in the tank, so they never changed shells. We have two and one was missing for several months, shell and all. He reappeared one day about two weeks ago, but the other one was missing and his shell was empty. I thought he was molting without his shell. What could have happened to him? Could the one that was molting for so long actually have died and my other guy found his shell? Should I dig around in the sand? I actually just purchased a new aquarium for them and left the empty shell in the old home in case he reappears.

Sometimes streakers will bury themselves to hide. But it's very dangerous to be without their shell for long, so yes, I would definately dig for him.
When you find him, (if he is naked and ok otherwise) very gently handle him, maybe using a towel (which is easier to use than your hands, and less risky, trust me) then, dip him gently in a bowl of lukewarm fresh dechlor water to wash him off, and place him in a small plastic container with a couple proper shells (wash the shells out as well) cover the container, or maybe put a lid on it, and put a towel or something around it to keep it dark. Put the container back in the tank (to keep it warm and humid, unless you have another warm and humid place to put him) for 10-20 minutes, check up on him and see if he's re-shelled. If he has not re-shelled on his own, you'll have to manually reshell him, which is tricky, but it has to be done. Manually reshelling is basically placing the crab into the shell yourself. An easy way to do this is to prop the shell up on something with the hole facing up, then, with his body on your palm with his abdomen hanging over the side of your hand, gently uncurl his abdomen with your finger and slide him into the shell.
Good luck I hope he's alright!