This began what we call the "Age of Faith". The Bishop of Rome, who we now call the Pope, began to assert authority over all the bishops and patriarchs in the Christian faith. Political authority was weakest in the West, so the Pope began to gain real power. At the same time, governmental positions were either taken by the invaders, so the only road open to men who wanted strong careers was in the Christian Church. The number of capable people working in the church and not in government further eroded governmental authority, and the church became the de facto authority in the West.
There were still cities in Italy, but it was broken up into city-states. The Iberian peninsula was conquered by the Islamic Moors. Britain was overrun by the Anglo-Saxon tribes. Most of what are now France, Germany and Eastern Europe were disunited.
One tribe, the Merovingian Franks, began to unite the warring tribes. Clovis I reconquered much of the old Roman Empire. The Christian church arranged his marriage to a Christian Princess, and he converted. He then required that all citizens of his empire convert to Christianity.
A serious problem with absolute monarchy is that the strength and unity of the state depends entirely on the strength of the monarch. The Roman Empire lasted for a long time, even with some terrible emperors, because the basic strength of the empire lay in its laws, roads and Senate. The Merovingian Franks had no such organization, and after Clovis the strength of the family waned.
The Christian church grew stronger; most reading and writing were conducted in monasteries, which were self-supporting religious communities where many went to escape the turmoil of the time.
The Benedictine order is a good example of someone from the gentry going into religious life. St. Benedict, founder of the Order, came from a noble family, but elected to go into monastic life. His "Rule", which sets down the guidelines for his order was adopted by many others and is still used in many monasteries today.
If you have time, I recommend reading it. http://www.osb.org/rb/text/toc.html