Alright, this is a topic that has been talked about for years and I feel there are an awful lot of misconceptions about what actually happened. These have not been helped by the sort of faux-historiography that gets pushed in the MSM. And as cool as it is that Mel Gibson made a movie totally in Maya and concerning the events of the Late Classic period, I'm afraid it did nothing to help lift the veil of ignorance that generally lays over this topic.
The collapse refers in particular to the rather disastrous downfall of the Maya lowlands centers in roughly the 9th century AD. Now some people have claimed this wasn't a collapse because people continued to live there after this time, and to use the sites of the Classic period as campsites and ritual centers and whatnot. Well, that's idiotic because when a whole way of life disappears and the land is >90% depopulated from what it was, that's a collapse in my book.
The Maya god-king system did suffer a catastrophic decline during this period and this completely eradicated the previous way of life that had defined the Maya. It's true that Maya civilization continued in the northern lowlands, but this is a half-truth because it was not the same as before. The Puuc centers were significantly different than anything in the south. The writing system that the southern lowlands used to such great effect seems to have been lost or drastically reduced. The architectural styles are quite changed. And what happened after this period was even more of a change than before. The most famous Postclassic site with Maya associations, Chichen Itza, shows a very different set of traits, many of which obviously are linked to Central Mexican traditions (feathered serpent cult, colonnades, chac mool reclining figures, etc).
So why did it happen? Well the early ideas about this have pretty much been proven false. Mass violence and warfare doesn't cut it. Maya civilization was always warlike, and there is no evidence of mass destruction by warfare. Hurricanes, disease, other natural disasters are likewise bunk ideas. The only idea that holds any weight as of now is the climatic change hypothesis, which states that a long period of drought added to the considerable societal tensions that already existed by the time of the Late Classic was decisive in undermining and destroying this culture. T
he Maya region's population was in the tens of millions and these people were mostly eating maize. This lead to health problems. It also lead to environmental degradation. Meanwhile the state and its administration continued to grow and overburden the local populace. There were considerable stresses inherent in Late Classic times. Then imagine a 50-year drought topping it all off. The Maya god-kings were supposed to be the intermediaries between the cosmos and humanity, and if they couldn't effect changes in the rain pattern that life depended on, what good were they? This was the final straw. The undermining of the god-king system combined with all of the aforementioned problems and a consequent agricultural collapse was what happened.
I hope this has been illuminating and please feel free to add anything relevant to the discussion.