Aquarium snails dying?

ok so my tropical tank with my betta and 5 neon tetras and a few plants and a snail, has killed my ramshorn snail and now my apple snail is sitting, fully extended, not doing anything for the past few hours... he did this before and he was fine. should i check the ph? that's the only tester i have. what do i do to help my poor little snails?