Belgian historian, Henri Pirenne asserted that early Frankish barbarians under the Merovingians actually maintained the Roman customs, laws, and institutions that allowed the Western Roman empire to continue to maintain its influence over trade in the mediterranean. Pirenne further asserts that Arab expansion in the 9th century cut Western Europe off from maritime trade in the mediterranean, effectively transforming Frankish Gaul from a connected economy of exchange into a closed economy of consumption and feudalism. Do you find Pirenne's claims legitimate?
Pirenne is at this point incredibly outdated (he died in 1935 after all) and aside from the standard "Pirenne was wrong" moment in contemporary works (and virtually every economic history has one) is largely ignored.
The idea that the Carolingian period was economically stagnant has been discarded as more attention has been paid to both Mediterranean and North Sea trade. It is not recognized that Europe was never a "closed" economy as such, and in fact much interest is now being focused on the ways in which Western Europe encountered and was connected to the wider world via trade, travel, diplomacy, etc.
The standard works you want to look at are McCormick' Origins of the European Economy and Richard Hodges' Mohammed Charlemagne and the origins of Europe: Archaeology and Pirenne thesis (really anything by Hodges will be of use)
edit: As to why Europe become "feudal" (and what that actually might entail) and when this occurred, well that in of itself is a whole other can of worms that I am going to chicken out on and hope that one of my fellow flaired who spends more time post 900 will pick up the slack :)