is it an apple/mystery snail?
i would try touching it. like on it's little trapdoor. if it pulls back in, then it's just hanging out bein cool. if there's no response, it's probably bedtime for bonzo. mine do that dormant thing off and on all the time, no biggie.
i don't think though that there's air in its shell keeping it from getting back in.
also, if they lose the trapdoor for good they're most likely dead.