What I know

After the Romans were gone, the protection that they had once provided was gone and so most of the trading stuffs were stolen. And the Roman Empire fell, so the money was no more. They had to find a new money system. At first they just traded stuff. Like 'I give you a sheep, and you make me a boat.' Another thing is, when the Roman Empire fell, the center was reestablished by the Catholic Church, so that was a big difference. More and more people started to depend on the the Church more than any other people. The economy also got bad, because there was no cash coming in their pockets, some was left with nothing to trade with but maybe a few pieces of money that doesn't mean anything after the Empire fell.