There were several reasons for the fall of the Roman Empire. Each one intertweaved with the other. Many even blame the initiation of Christianity for the decline. Christianity made many Roman citizens into pacifists, making it more difficult to defend against the barbarian attackers.
In 476 C.E. Romulus, the last of the Roman emperors in the west, was overthrown by the Germanic leader Odoacer, who became the first Barbarian to rule in Rome. The order that the Roman Empire had brought to western Europe for 1000 years was no more.