I got one that showed up a few days after I put in a new plant. Did you make an addition to your aquarium any time recently?
I moved the snail I found out into a spaghetti jar filled with water and let it live in there for a while just to make sure I didn't get a major population boom of snails in my aquarium. I gave the snail a small piece of sinking fish food once a week to keep it alive. After a few months, my aquarium didn't show signs of any more snails and I dumped the one back in. It's nice to have the snail since it keeps the algae in check. Keep an eye on your snail and see if any more show up bacuase they can really crowd your aquarium. (Some snails can reproduce on their own.)