It appears that the real demise of the Mayan Empire was a number of factors, including drought, warfare, and disease. NASA archaeologist Tom Sever used satellite images combined with archeological findings to piece together the most likely scenario. Using pollen trapped in layers of lake sediment, scientists learned that around 1,200 years ago, just before the civilization's collapse, tree pollen almost completely disappeared and was replaced by weed pollen. This indicates that the region had become almost completely deforested.
Without trees, erosion started carrying away fertile topsoil. The changing groundcover boosted average temperatures which dried out the land, making it less suitable for crops. In support of this, researchers find human bones from the last decades before the civilization's collapse showed signs of severe malnutrition. Rising temperatures would also cause a disruption in rainfall patterns. During the dry season, water would be scarce, and the groundwater was too deep to reach with wells. Dying of thirst became an additional threat.
"Archeologists used to argue about whether the downfall of the Maya was due to drought or warfare or disease, or a number of other possibilities such as political instability," Sever says. "Now we think that all these things played a role, but that they were only symptoms. The root cause was a chronic food and water shortage, due to some combination of natural drought and deforestation by humans."
— Steven Drennon, answer posted five years ago