What type of groups replaced the Romans as rulers in Europe in the early Middle Ages?

When Roman power collapsed in western Europe, it continued in most of the Balkans, Asia minor,the near east and Egypt until the 15th century in the guise of the Byzantine empire; though they always referred to themselves as Romans, Byzantine being a western term to differentiate this later period after the western collapse. Western Europe was overrun by Germanic tribes crossing the Rhine westwards and the Alps southwards. The Franks, Vandals and Visigoths conquered Gaul, the Franks stayed to rule it hence its modern name, France. The Vandals and Visigoths continued moving, crossed the Pyrennies and conquered the province of Hispania (Spain & Portugal) which the Visigoths then ruled. The Vandals continued south, crossed the Straits of Gibraltar to conquer north Africa as far east as Egypt and also invaded Sicily. Other tribes, mainly known as the Ostrogoths, crossed the Alps southwards to conquer Italy as far as the southern tip. The island of Great Britain had been under constant attack by raiding parties sailing directly from the coast of modern Denmark, North West Germany and the Netherlands for some time and thus had been protected by a Roman naval fleet spread along the east coast. When the Roman army and fleet was withdrawn from Britain to defend Italy the Germanic tribes led by the Saxons, Angles and Jutes were able to conquer, enslave or evict the Romano-Celtic population and settle themselves on the land. They succeeded in pushing the remnants of the Celts to the extreme western and northern limits of the island thus giving their names and language to the major part of it;England=Angle land, Wessex=west Saxon, Essex=east Saxon etc. Welsh is an Anglo-Saxon word meaning foreigner. The influx of refugees, Britons, from Britain to north west Gaul, populated by their allies and related tribes led to the region becoming known as little Britain or in french, Petite Bretagne and now simply Bretagne. Only in the very north of Britain did a celtic population expand when an Irish tribe from Ulster, the northernmost province of Ireland, invaded and then migrated in to the area just north of Hadrian's wall.This tribe was the Scots who gave their name to all of the northen third of Britain. This list of names of the Germanic/Teutonic tribes is far from exhaustive, many other well known tribes such as the Allemanii,Batavi,Burgundi,Dani, Frisians, Gepids, Lombards, Marcomanni,Suebi were all part of this mass migration.