Roman emperors had, at their peak, been the rulers of western Europe and the most powerful individuals in the world. This influence peaked about 200 years after Julius Caesar was killed and slowly declined for another 300 years. By the time Odoacer deposed the Emperor Romulus in 476, there was not much left of the Western Roman Empire, and while they continued to call themselves this, Rome was broke and exerted no control over its old territories.
The Medieval period--the buffer between the ancient and modern worlds--is generally considered to be from the fall of Rome to the discovery of America. The first part of this, roughly from AD 500-800, is known as the Dark Ages, which ended with Charlemagne and the rise of the nation/state, specifically the Holy Roman Empire and the groundwork for what would become France, Germany and Italy.