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 a big blow. Time had done its damage to the many buildings in Rome, and to add to that, Nero's fire finished nature's work; it took out almost the whole city, and was unstoppable. Each new building was another log thrown on the flame. The fire had razed the city, and seeing as no one was spending money, and the treasury was ransacked, no one had money to rebuild the city. The empire became so large, that all trading, and all money became so unstable that it crashed right back to where it started. (http://www.acs.ohio-state.edu/history/isthmia/teg/Hist111H/issues/rome2.html).
At one time, Rome possessed one of the most powerful armies in the ancient world. Many theories state that the army slowly declined during the last years of the empire, causing the fall of the Roman Empire. One theory states that a massive Roman army was put together for the western part of the empire, and it slowly lost men from Barbarian attacks, civil wars, and splitting up to fight multiple battles.