Im a snail lover. I own a large apple snail. We bought him the size of a pea and he is now half dollar size and still growing. I also spend a great deal of time reading.
If ever in doubt pull the snail out and touch his door if he moves he is alive. A dead snail smells really bad and you will know it is dead. They start to stink rather fast.
Snails have unusual behavior. They can float for extended periods of time as if playing dead they can even close up shop and shut that door tight and go to sleep. In fact there are reports in tanks of snails doing this for a few weeks to a few months.
Touching their door and smell test is the only sure way to know it is still alive.
Snails also like to become inactive and close up when the water is not the best. As joannde stated I have noticed that with my snail as well. Those NitrAtes really bother him and he will not be as active.
Mine is healthy and he still sits on the bottom with his door barely cracked. In fact as I type this he is doing it sitting on a fake plant. He just likes to hang out. I know he is very active so I know he is not doing it out of illness.
What are you feeding your snail?
Also snails are very well known for "playing" mine spent two days straight playing in the airstone in my sisters 10gl tank. He was hitting the glass and everything just going in circles. AKA bubble surfing. We had to keep checking he was still alive and he would go right back to it. In my 55gl tank he walks along the bubble curtains several times a day. He lets air go into his shell and floats about. They just do odd things!
is an amazing site for information on snails. The forums are also full of information. That is how I have educated myself so well on my little guy.
Let us know how he is doing!