The fall of the Western Roman Empire used to be thought of as a collapse of civilization, leaving a great void into which swarmed barbarians. Out of the rubble, the people of Europe slowly rebuilt civilization into what we think of as the Middle Ages. The reality is the fall of Rome was a process. Roman rule was replaced by something else, something local and indigenous.
Zman’s ‘Essential Knowledge’ posts are full of links to good books for your continuing education.