I answered a related question a while back about the Gallo-Roman aristocracy in Gaul. You may find it helpful. I am happy to answer any follow up question you or others may have. Basically, Romans largely remained in the regions that had once been part of the Western Empire. Life certainly changed over the course of the fifth and sixth centuries, but they did not physically disappear. The disappearance of 'Roman' as an identity is related instead to changing ethnic categorizations and alignments during the formation of new political communities in the post-Roman West.