Is My Mystery Snail Dead / Dying?

[Doomchibi]

I have a 2" black mystery snail in my 75g freshwater tank, and up until now he has been perfectly fine and all over the place. I just went to feed everyone and I found it at the front of the tank, partially out of it's shell..? I reached in and poked him, (he did not respond) and moved him a bit and it seems like he is a little bottom heavy, as in if you turn him over the upper part will turn back upwards and almost float a tiny bit. I broke up an algae wafer and dropped some next to him and turned the light off (which I do after every feeding), came back about 20 minutes later and he had not moved. I really do not want to have to take him out and smell him, but I also don't want to leave him in long enough to decay if he is indeed dead. Should I give him a day to see if he moves or is it possible he is okay and resting? The store I bought him from had snails that would sometimes sit slightly out of their shells as well, but I have never seen this one do it personally. I have had him for about a month. My first thought was maybe he was just fat and decided to "let it all hang out" literally? That's what i'm hoping anyways. The part that is out is completely intact as well so I highly doubt any of my fish were going after him. I saw him last maybe 10 hours ago on the tank wall, happy as can be. I have a nerite snail as well and he seems to be perfectly fine, in fact he is taking the opportunity right now to go after the algae wafer I put in for the mystery snail. The only thing I have done slightly differently lately was add plant food, which I usually do every week but I forgot to last week.. I doubt that would have caused a problem though. Any help would be appreciated

 
 
I apologize if this is in the wrong section, I figured people in the invertebrate section may have a better idea which is why I posted here but feel free to move if you feel it would be better somewhere else.
 
Edit... I just looked at him again and it seems like he may have turned a bit..? If he is alive, it seems very odd that he was slightly floating and sitting like that.. Is this kind of behavior normal? I am hoping he is not sick.

 

[lrhodes]

The only snails I had that did that turned out to be dead... but I didn't bother to take them out, because like you I was wondering if they were alive. (Of course my water conditions were terrible when I had them! So that's probably why mine died...)
Idk. Give it a day?

 

[Essjay]

There is an easy way to tell if a snail is dead - lift it out of the water and sniff it. A dead snail smells horrible. If a snail doesn't smell, put it back in. If it stinks, it's dead.
Take a very small sniff - that smell sticks to the inside of the nose for hours

 

[Doomchibi]

He seems to have completely disappeared from where he was yesterday. I checked all around my driftwood, plants, near the filter intake, and all along the top of the glass that is out of the water. I think somehow he must have been alive and decided to hide somewhere.. My tank is completely covered so he could not have crawled out. If he is OK though, his behavior yesterday seems very odd and he might be getting sick. Hopefully I find him soon.  

 

[Ninjouzata]

My snails do this every once in a while, was watching one of them earlier tucking itself in actually
Do keep an eye out though, I had one that I figured was fine and finally got worried, checked.. 

 As has been said, it will smell, and it will be very obvious it's dead.

 

[choldread]

Well have to say the sniff test is a good indicator. Just bought my first two and they were questionable from the start. Sure enough, a quick sniff and they were gone.

 

[betta fish]

Well it's good that it has moved somewhere else that you can't see as it obviously has the energy to do so! I find before any of my snails die they just stop moving and go into a hibernation partially buried for a few days then I just find an empty shell, I have lots of shrimp and trumpet snails plus a large goldfish so the snail doesn't pollute the tank much before it is dealt with, it sounds by the floating behaviour that there was just air within the shell that needed to come out, the behaviour suggests this too, they often get a bit of trapped air when they surface, don't worry about this it's perfectly normal, obviously if you are continually noticing the snail becoming less active than before and it doesn't come out of it's shell much when it is active then it might be on it's way out, but the fact it has moved from the place you last saw it is a good sign