Dynasties rise and dynasties fall and have done so throughout the ages. Borders change and countries become whole new entities. Governments fail, new governments take their place and humanity repeats the same mistakes over and over and over again. Some civilizations are lost forever. Some simply integrate into the civilization that conquers them. Change is inevitable. Sometimes good. Sometimes bad. But ALWAYS inevitable.

Many fell simply because they didn't have an effective way to go on when a strong leader died (as in Mongolia), many would break into smaller countries because rather than having one successor, they would share the power between several (such as late Rome and Alexander's empire).

The mayan calender has been misinterpreted by many, sometimes for personal financial gain. The calender does not predict anything happening in 2012, it just starts all over again, sort of their version of 2000.

In truth the 20th century was probably the closest we ever came to the dark ages and we got through it.