I've answered this question or similar ones before, most recently here. The tl:dr for that answer is that the collapse of the Roman Empire is a slow splintering of Imperial provinces and steady erosion of power which is heavily impacted by a major economic recession. It's really down to the Church, with the unifying theology espoused in works like Augustine of Hippo's City of God, that much legacy of Rome survives. As the Imperial economy collapses, so do the vast networks of logistics, trade and manpower/slavery which propped up the Empire. People have to increasingly rely on local subsistence agriculture for food, buildings and cities become increasingly prohibitively expensive to maintain and repair, and government falls back on pre-Roman structures.

If you want a lot of in-depth information on this subject, there was an awesome AMA about the "Dark Ages" awhile back that you might find interesting.