The shell will be empty if you have any kind of hermits or scavengers that would have eaten them after they died. Otherwise, the foot of the snail and the door to the shell will be hanging out and the snail will not respond to touch.
What kind of snails are these, do they have a shell that looks like a "conehead" (Astraea snails)(Sometimes even called turbo snails) or is it different than that? You're going to lose a few of the conehead types occasionally because they will fall down and they cannot right themselves, and sometimes get pulled from their shell and eaten by hermits before the aquarist (you) can rescue them again.