How was Europe, if at all, reunified after the fall of the Roman Empire?

Europe was plunged into Dark Ages after the Fall of the Roman Empire. The continent was in shambles with no central rule. Charlemagne's system of feudalism that began and was the basis of Medieval times (the Middle Ages) was the best unification of Europe however still much of the continent did not adopt the system. This lack of uniformity however was not necessarily bad as it aided the development of the diverse European continent we see today in the 21st century.