The fall of the Roman Empire is one of the most confusing issues of ancient history, especially with so many possibilities. Rome definitely fell as a result of its own weight crashing down upon itself, but the possibilities are endless, since nothing has really opened a door to the past. Three main possibilities stick out beyond the rest, they are: the economy collapsed, the military collapsed, or gradual transitions were made that slowly caused it to become like a different civilization.
The economy in Rome was excellent until citizens started hoarding their money because trade was declining. Money almost stopped circulating in western portions of the empire. Barbarian tribes were coming into cities, and in addition to stealing goods, they were stealing from the treasury, which makes money just that much more rare. The west was not trading at all, and money seemed to be at a total standstill. Another economic-related theory states that Rome's infrastructure was taking