The Maya civilization developed in an area that covered southeastern Mexico, all of Guatemala and Belize, and the western portions of Honduras and El Salvador. It lasted for more than 2000 years, and at its peak there were around 13 major cities, the largest of which had an estimated population of 100,000.
Almost the entire civilization disappeared during the 'terminal classic' period of AD 830–950. There are now more than 80 theories which attempt to explain the collapse, but there is no common agreement amongst archaeological experts.
The Classic Maya collapse is one of the greatest unsolved mysteries in archaeology."
Source : Wikipedia