One theory of how the Mayan civilization declined is famine.
It starts with the crops. The Mayans used the slash and burn technique for farming, which isn't sustainable for a long time. This could cause bad harvest.
There isn't enough food for all of us. What will we do?
We're all going to die!
The Mayans would've ran out of food and starved to death.
However, famine isn't the only theory of how the Mayan civilization fell apart. Another, more plausible theory is warfare. Since there were a lot of resources constantly being expended, the Mayan civilization needed a lot of materials. Once their own resources were running low, the Mayans may have gone to war to get more.
The Mayans had a big army, but also had many other small tribes fighting against them. They built defenses and weapons, which were discovered years later in the ruins of the Mayan Civilization.
Win or lose, war costs lives and materials. Over the years, the Mayan armies must've shrunk and the civilization may have been conquered and ruined.