Many different incidents contributed to the downfall of the Roman Empire. Such factors included the faltering military and political standings, as well as social and economic ruin. Slowly but surely, this once omnipotent empire lost its power to its surrounding countries.
Politically, Rome was devastated. Most were reluctant to rule over such a vast territory. When Diocletian came to power, he split the empire into eastern and western halves. But all this accomplished was civil war. Constantine took up the throne and moved the capital east to Byzantium. With most of its power in the East, the Western empire fell first.
In a social sense, the Rome's people were in confusion. They no longer had confidence in their empire and this was a cause for disloyalty and corruption. No one expressed any interest in public affairs. There was also a distinct contrast between the rich and poor, which brought about numerous rebellions. .
The economic situation in Rome was in ruins. The rich, who spent so much on luxuries, Had drained the empire of gold, silver, and precious metals, and because the expansion had stopped, there were no new metals to be found in new conquered territory. Constant wars were very costly and didn't bring in useful spoils. The farmland was no longer fertile, and hostile enemies and pirates had put a stop to overseas trade. The economy suffered from greatly from inflation too.
The military was almost non-existent. They had trouble recruiting Romans who were by now disloyal. The military hired mercenaries who simply had no patriotism and likely, weren't willing to die for the cause. There wasn't enough money to finance the militia, and there were also constant threats from Northern European tribes.
All of these elements hastened the downfall of the Roman Empire. But the invasion by Germanic tribes and Huns, as well as conquest by other invaders, was simply "the straw that broke the camel's back".