What happened to the Mayan civilization, and why did it vanish? Was it an abrupt ending, or a general collapse that happened over a span of years or even decades?

This was, in effect, a collapse but took place in stages. There was a corruption from within of the culture and the individuals taking part, as well as outside pressure from European civilizations adding greater negativity to the mix and encouraging and hastening a turning point where there was a decision simply to not return. This was a recognition that things had gotten out of hand because of the interloper manipulations. The Mayan people, who were originally quite spiritual and had a balanced view of life and interpersonal relationships, gradually became corrupted like other human peoples and this led to increasing depravity in their governance and in the expression of their religious rituals, as with human sacrifice. This became quite a sad chapter in human history but is almost universal across the planet, that one by one, groups of humans of any size take a turn for the worse and there is a degradation of their culture, their habits, and routines with the choices becoming more and more self-serving, harsh, and regimented with strict rules and great penalties for any deviation. This has caused the downfall of all the great empires through history. In a sense, they collapse under the weight of their own misguided power and domination that has attempted to suppress their citizenry. Those are many demonstrations of the difficulty that the interlopers are having, to ride herd on humanity and effect a final solution. It is not going smoothly at all and this is causing increasing consternation and confusion amongst the members of the Extraterrestrial Alliance. This can work in your favor by encouraging a withdrawal, simply to be done with this mess and all of the hard work required to ride herd on the many different projects going simultaneously to create this death of a thousand cuts that is underway. So the disappearance of the Mayans is, in effect, a kind of ascension by virtue of a decision to not descend—once back in the light, they chose to remain and not lower and degrade themselves with a further tour of duty in the difficult earth plane, given their inability to maintain the purity of their culture against the opposing forces of corruption.