From Wikipedia

Who Killed The Maya

The Classic Maya Collapse refers to the decline and abandonment of the Classic Period Maya cities of the southern Maya lowlands of Mesoamerica between the 8th and 9th centuries. The Classic Period of Mesoamerican chronology is generally defined as the period from 300 to 900 A.D., the last 100 years of which, from 800 A.D. to 900 A.D., are frequently referred to as the Terminal Classic. The Classic Maya Collapse is one of the biggest mysteries in archaeology. What makes the Classic Maya collapse so intriguing is the profound heights reached culturally by the Classic Maya before the collapse; and the relative suddenness of the collapse itself.

The highly-advanced Maya centers of the southern lowlands went into decline during the 8th and 9th centuries and were abandoned shortly thereafter. Archaeologically, this decline is indicated by the cessation of monumental inscriptions and the reduction of large-scale architectural construction.

Some eighty-eight different theories or variations of theories attempting to explain the Classic Maya Collapse have been identified. There is no universally accepted theory, though the drought theory is now gaining momentum as the leading explanation.