I have found guppies to be extremely hardy, until they aren't! My tank contained guppies who were all descendants of a small group purchased about 4 years, and who were lively, busy, and healthy. But, about two weeks ago, their numbers suddenly dropped. All of the babies vanished. A week later I had no guppies left at all. The other fish left in the tank continued to act as if in very good health. I have now restocked the tank with tetras, and they are doing fine. No fish in the tank are big enough to eat guppies, other than babies.
This is about the third time this has happened for me, over the past 30 years or so. I have no idea why it happens.