That is really strange and it looks like the poor snail is still alive It will likely die in a little while w/o the protective shell. If you want to euthanize it that would a wise choice, otherwise I'd at least gently place it in a floating dish of water in the main tank to keep it close to the surface for air, add a bit of food to see if it eats.
Does the shell have any damaged areas or holes in it? Do you know if the snail suffered any trauma recently (fallen on the floor, recently bought perhaps?). I've no idea what would cause a snail to lose it's own shell like that.
I've had him for a couple months now. He had a crack and a hole in his shell. He had not fallen out of the tank and I don't know how the cracks had gotten there. My guess is that there wasn't enough calcium in the tank and he was growing but the shell wasn't, and that was causing the shell to break.