Read this section on my site:
http://www.fishpondinfo.com/snail.htm#deadApple snails (and other species sometimes) will hole up in their shells if they are not comfortable (new home, water quality problems, someone picking on them, etc.). Aside from waiting and making sure the water quality, temperature, and other animals are to their liking, you just have to wait until they decide to come out. As long as the trap door is shut tight, the snail is alive. It opens up when they die. I hope your new guy decides to come out soon!