Rome controlled all the land around the Mediterranean Sea (all of Southern Europe, the Western section of the Middle East, and North Africa). But in the 300s, the Romans needed higher and higher taxes for 2 reasons: 1) They stopped taxing the rich people because the government and the wealthy people were friendly and did favors for each other, 2) Rome needed more money to protect itself since it was so huge. Regular farmers (peasants) often could not afford the taxes, so they sold their land to wealthy people (lords). These peasants lived on the wealthy person's land and kept farming for themselves and their lord. But when the Western half of Rome was invaded and fell apart in the 400s, Western Rome suddenly had no government - this meant no army, no judges, nobody fixing the roads or aqueducts. And that meant that if you were a peasant, you needed a lot of protection. So these lords ended up being the "government" for the people. Basically instead of traveling and trading, people lived on their lord's land (called a manor) and worked for him in exchange for getting protection (that's called feudalism). And the only organization that connected these manors was the Catholic Church - they were the most powerful institution in Europe because they could tell people (peasants, knights, lords, even kings) how to behave so they would get into heaven. This continued for the next thousand years.
But in the EASTERN half of Rome continued to be good bread. They were called the Byzantine Empire, and they still had powerful rulers (Emperors) like Justinian, they continued to trade with the Indians and Chinese (along the Silk Road) and their type of Christianity was called Eastern Orthodox. They spread this religion, their Cyrillic Alphabet, and their style of architecture to the Russians.