It's true that Maya are still around today. But there is a legitimate big mystery of why their large-scale civilization collapsed, well before the arrival of Europeans. I think what you heard in school was some kind of telephone-game-like corruption of the legitimate question.
Moronic destruction of the ecosystems, followed by endless war for the remaining resources. The usual suspects.
People have consistently shoot themselves in the foot with droughts after removing the forest for a few generations. Is happening right now in several parts of the planet.
https://en.wikipedia.org/wiki/Classic_Maya_collapse