I think Hawk, it was a cattle prod. jk. : ) I'm with Hawk though, it would be any of the above. I know this might sound stupid, but it could have sustained an injury. I know one of my turbos fell off of a hose or something and cracked his shell really bad, he's fine but I'm sure if he fell just right it would have been enough to seriously injure him. I think I'd lean more towards natural causes. I think if your ammonia,nitrites, nitrates ect. was really high something else would have died before the snail. How do your other animals look? Are they stressed?