One of my new Waterhouse snails (very similar to apple snails, a large trapdoor species native to Australia.. ) recently was exposed to a toxin of some sort in my daughter's betta tank - the fish is just fine but all her inverts are dying - we think there's traces of copper in the substrate. Anyway...
My snail did a very similar thing. I rinsed him off in conditioned water, plopped him quickly back in the large tank and left him alone. It took a couple of days for him to recover, but he did.
Lots of very clean water. Not being stressed while he heals (leaving him alone worked for mine, anyway). Don't worry about food - he'll eat when he feels better and can get about.
Sorry about your snail! I hope he recovers.