The fall of Rome involved the disintegration of the Roman state; the collapse of long-distance trade; the disappearance of mass-produced pottery, coinage, and monumental architecture over large areas; declining literacy among commoners and elites; great insecurity of life and property, and demographic collapse. The process was drawn out and played out differently in different regions. In the Middle East, central government supported by taxation continued; in the West it disappeared. The nadir in the West was perhaps the tenth century. We might set the turning point at the battle of Lechfeld (955): a last set of invaders off the steppes, the Magyars, was defeated by the Emperor Otto, and then adopted Christianity, gave up nomadic marauding, and settled down as feudal lords in Hungary.
The fall of Rome illustrates a general lesson. The overall trend of history is for more complex societies to replace less complex. (Important note: “more complex” is not the same as “nicer.”) But the process is an uneven one, in part because military effectiveness is only loosely coupled with social complexity. Tribal peoples with states next door often react by developing states of their own, partly to defend against their civilized neighbors, partly to prey on them. The resulting societies – no longer tribal, not really civilized, but barbarian – have sometimes been more than a match militarily for their more complex neighbors. In Europe, the result over nearly a millennium was a great leveling process. Rome declined under barbarian assault, while state organization, class stratification, and Christianity spread eventually as far as the Slavic East and the Scandinavian North. (See Peter Heather’s Empires and Barbarians.)
By the end of the first millennium, Western Christendom had some consciousness of itself as distinct from the Islamic world; this would later help motivate the Crusades, but it would never be enough to spur unification. Much later, in the twentieth century, Europe would be divided by a different set of meta-ethnic frontiers, centered on the clash of ideologies, rather than civilization versus barbarism. But that’s a story for later.