This question is old as the Mayan civilisation. And not even in the 21st century we can't precisely say what happen to them. Maya culture at the time was the biggest and wealthiest civilisation on the earth. They were occupying Central America. Their cities had over 100 000 people. For that time, that was a huge number. Maya people were very spiritual. They believed in many gods. But they were also very brutal at God worshiping. Some proofs say that they had human sacrifices in the name of God.
Who Killed Maya
But what could happen to so powerful civilisation? The problem is that this civilisation vanished in less than 100 years. So what could go wrong in so short period? Why did the Mayans disappear? All cities were abandoned. There are some proofs of slaughter. But also some evidence of heavy droughts. Is it true that these events made Maya vanish in so short period? There are many speculations and many proofs.
Maya civilisation was destroyed in the war. At least that is what Nikolai Grube says. He is German epigraph and he dedicated his life to try and understand Mayan hieroglyphs. In his documentary "Who killed Maya?" he provides proof of genocide. All his facts come from stone hieroglyphs he found in Central American jungle. The central character in his theory is Lady Six Sky. She is a war princess. She was the Mayan war leader. Since she was female in males world, she needed to be strong. And to remain strong she did not obey any war rules Mayan people had. And this lead to genocide. There were massive wars all over the land and people were just running out from the cities. In less than 200 years everything vanished. It is possible this happen in massive war. It is possible that Mayan people destroyed them self in genocide war. Such spiritual people vanished because of stupid wars.
You can draw a line here, that even strong civilisations like Mayan culture can be destroyed in the war. And we always have this little wars in modern time. The USA vs. Rusia and now a middle east crisis. Can these events trigger a real world war 3 and possibly the end of the world for our civilisation? If Maya culture got destroyed in the war in less than 200 years, there are high chances our world could come to the end as well. That why we need to fight for the peace.
You can see all details about this war theory in "Who killed Maya" over at YouTube:
Another theory is that Mayan land was hit with massive drought. There are also many proofs that backup that. Maya had big cities with over 100 000 people. That people need to eat. So food was very important to Maya. If many people would starve to death that will most probably spread disease trough city. And some huge epidemy could end the Maya civilisation.
Did drought destroy Maya?
We can draw a parallel line here as well. Droughts and starvation are already happening in modern time. As well we have many epidemics. A Recent one was Ebola epidemy. This kind of events can kill a lot of people in short time. But a reason for our drought can be found in pollution. We are polluting our planet and it is just matter fo time when nature will fight back and send famine and heavy droughts.
So if you live in the country, nourish your land. Grow your food and don't use industrial fertilisers and pesticides. Maybe one day this earth can save you.