Most likely it's dead. They will often play dead, but if it hasn't moved from one spot in days it's dead. Just pull it out, and smell it. If it smells really foul it's dead and decaying. (ammonia central) If not put it back in the tank in an obvious spot, and come back in 2 days. If it's still there it's dead.. . PS- Note that a mystery snail can some times float when it's alive.