I have a 10 gallon tank I had 3 guppies and some snails. Only 2 adult snails and some babies. I had 3 guppies the night before and the next day one of my guppies disappeared. This has happened before and I’m really frustrated and sad about it. Please help.
The most reasonable explanation is that it died and was eaten by the other fish / snails. Note that some snails are carnivorous. Many fish are cannibalistic if the opportunity arises.
Some leftovers of the missing fish might be found inside the aquarium, if you look carefully. Or in the filter.
Some fish have the idea to jump out of the water. With this occasion, they might even jump out of the aquarium entirely (that is why aquariums have lids). Look around, even behind furniture. You might find it. Quite probable, it will not ever move again by itself.
Ask your family. Maybe they collected it dead from the water and disposed of it.
Funny or not, I once found in the "aquarium" (actually bowl) the skeleton of a fish (possibly molly or something similar) - the way seen in cartoons (head, spine, bones, tail). It died, and the other inhabitants were happy with the diet change.
Update: if the aquarium does not have a lid (or if you find the lid removed "without explanation"), and at the same time you own a cat, then it is the time to have a serious chat with the cat :)