How can I tell if my mystery snail (apple snail) is alive or not?

My ivory colored snail hasn't been doing anything for the past few weeks. Its been in the same spot too. I just put it out in a little cup filled with water under a lamp to warm it up because I was told that they can die if it gets too cold. Its closed up but is far inside its shell and its shell is starting to change colors to blackish grey. What's wrong with my snail, and is it dead??