The Roman Empire fell in 476 when the military overthrew the last emporer. The bureaucracy stumbled on for a while as though nothing had happened, but eventually taxes stopped geting collected.
The legions that had kept peace in the empire didn't get paid, so they went home or married into the barbarian families resident in that portion of the empire where they were serving. Let me give you an anecdotal lesson into what happened.
When the empire still stood, a Roman judge issued a decree in a civil case that injured a barbarian family. One of the barbarian menfolk killed the Roman judge. A day later the local Roman legion swept in, crucified the menfolk and sold the women and children into slavery. Roman law prevailed.
After the fall of the empire, the same thing happened, but there was no retribution. The day after the judge was murdered, the local barbarian clan chief and his men swaggered around town. There was a new boss, and everybody paid protection (taxes) to him.
Multiply this on a grand scale, and you see the highways taken over by bandits, towns walling themselves up for protection and trade coming to a halt. Add to the mix the takeover of the Mediterranean by Islamic navies a few centuries later, and you have the Dark Ages.
For protection, towns and landowners hired former legionnaires to protect them. These people were essentially thugs on horseback and didn't become knights until the Church got its hands on them and promulgated a code of conduct that was later known as "chivalry". (Comes from cheval, the French word for "horse".)
People without the means of self-protection sold themselves to the large landowner of the region, and thus serfdom was invented.
In the dying days of the empire, the gold and silver coinage had been adulterated by base metals or clipped to the point where their worth was questionable and not easily able to support commerce. Barter replaced money.
With barbarian tribes moving into formerly Roman areas and the Islamic threat from the east, the Papacy attempted to recreate the Roman Empire under Charlemagne. That helped by preventing the Muslims from crossing the Pyrenees from Spain. It also Romanized the barbarians by converting them to Christianity.
I'm drastically condensing what happened so that it fits into a reply rather than a historical tome, but you get the picture. Monetary collapse led to societal collapse which in turn led to the dissolution of Europe. It wasn't pretty.
Thank you for taking the time to write 104; I find it invaluable.
I think the same thing could happen in our future.
Wow.....looks like tomorrows news .
Thanks for the excellent synopsis! Do you have a particular book in mind that would more fully explain what happened yet is not too dry?
Thanks for the ping Travis!