For centuries, Rome was one of the most powerful empires in the world. However, due to several issues in the mid-late 5th century, the empire collapsed and the city of Rome fell.
In 476 AD, the City of Rome is sacked by the barbarian uprisings, and the sitting emperor, Romulus Augustus, is overthrown and exiled.
As Rome was attacked from the outside, it was also collapsing from the inside. The constant state of war Rome was in led to economic troubles. The government was forced to heavily tax it's population, which led to the rich exiling themselves to escape tax collectors, some even set up their own independent fiefs. Additionally, as expansion of the empire halted, the empire could not make any more conquered people slaves. The empire's reliance on slave labor saw that the Roman's work force severely declined, and commercial and agricultural production suffered.
After Constantine made Christianity the official religion of Rome, internal conflict caused the empire to split. Constantine's empire became the Byzantine empire and survived for over 1,000 more years. The Western half had nowhere to expand, and their empire started to weaken.
The empire had also over-expanded, it's borders were so broad that administration could not be handled. It also spent too much money on the military, weakening the economy of Rome.