I'm not sure about the anatomy of a snail... but when people get a collapsed lung, the sac around the lung (not the lung itself) fills with air, causing the lung the be pushed aside. If that's what's happening to the snail, I would think that like another poster suggested, you could poke a small hole in the sac and see what happened. If they're like humans, once the lung reinflated, the small hole you created would seal itself.
Not sure if you want to try it, but it seems like a lost cause if you don't... or if you do and it doesn't work.
Good Luck!