My hermit crab looks dead and lost a leg but he might be molting?

He's not moulting. Loosing legs is often caused by stress. if he was moulting he would have lost all his exo. It would look like he is dead but it would be hollow. Kinda freaky the first time you see it.

If he is hanging out of his shell not movie it is possible he is dead. They often fall out of the shell when they are dead but not always. A strong fishy smell will be noticed. Try tapping his feelers and look for a response.

check the humidity and Temp and see if they are high enough. Has he been getting salt and fresh water treated for hermit crabs? And check for mites.

There's no need to worry! As long as you have proper tank conditions (especially proper temp and humidity), it's totally normal for crabs to do this. My crabs often bury themselves several times, for either a few hours or a few days at a time, leading up until they finally decide to molt. Depending on the size of your crab, it could take him a couple of days to a couple of months to molt. The most important thing is just to keep your tank conditions normal and don't move him or the sand around him. Don't be surprised if he comes up after a few days and isn't molted yet, it's all normal. You'll know when he's molted - his colors will be brighter, his claws and legs will have sharper points, and any antenna/legs/claws he might be missing should be grown back. Just let him do his thing. Relax, your crab should be just fine. :)

Try gently moving some of his limbs. Do they feel loose with no strength in them? Check the leg that fell off. If it's solid (not hollow), then your hermit crab has lost a leg.

Is the body sticking out of the shell? Try pulling it out (gently of course). If it is loose, but doesn't feel hollow like an exoskeleton, I think your hermit crab has passed away.

if he has a strong fishy smell then most likely he is dead

if not there is a very strong chance he is going under a surface molt, they become very unresponsive during this time and will even fall out of there shell so its best to remove him from the other crabs to prevent them from killing him

If your crab isn't moving at all ( try blowing on him) he is probably not alive any longer. And hermits crabs don't molt but they do move to a bigger shell whan they outgrow the ones thay are in. So if you get another one, have a variety of shells on hand for him to move into. I've seen some take soda cans for a home!

poor hermit crab, he might be stuck? Hold a mirror or glass to its face, fog, alive, none, then your poor crab is dead. :(

I think they come out of their shells easily if they are dead. If you pick him up by the shell and he falls out, he's a goner.

leave him there a little longer... if he starts to smell, hes dead. but those things loose their legs all the time... they just grow back.

Hold a mirror up to it's nose. If it fogs up, he's still breathing. If it doesn't, funeral arrangements are in order.

How long has he been in his shell?

Does he smell rotten?