Note: The common belief that the collapse of the western Roman empire led to the economic stagnation of the Dark Ages appears to be incorrect. The collapse didn't help matters, naturally, but according to Medieval Times "[t]he economic crisis started much earlier and could not be reversed either by the reforms of Diocletian nor of Constantine the Great." In other words, the western empire was already going through a several economic crisis when it collapsed.
The fall of the empire definitely led to a further breakdown in law and order which naturally affected trade, and the Roman monetary system seems to have vanished:
"Monetary economy gave way to the natural economy. The disappearance of the monetary system in Western Europe greatly affected both trade and progress of economy although the trade with the East did not completely disappear." It would take centuries for Europe started to recover.