The culprit for the death of roman civilazation these days is widely thought to be the muslims. They completely destroyed the roman civilization and tech on the south side of mediterranian and spain. that generally known and accepted. recent research has shown that muslim raiders flattened every coastal city on the north side of the mediterranian between 700-1000 ad.
What the Muslim breakout did was destroy Mediterranean trade & trade links. Such as cut off Roman civilization from Middle Eastern and Asian\Indian ocean trade, grain from Egypt, etc.