The Roman Empire's fall wasn't just one single event it was more like power shifting in my opinion.

There were a plethora of issues going on in Rome during the 5th century AD such as foreigners invading the frontiers, the army was stretched too thin across Europe, the Middle East, and northern Africa, and the empire being bisected.

For the first and second points, there was a constant issue in Rome with how they paid their armies. They were paid in coins, yes, but also inland which was taken during a campaign. This became a problem when Rome stopped expanding and had to protect their borders so that means soldiers weren't getting what they felt was owed to them. So that didn't help morale in the already tenuous situation of "barbarians" at the gates, especially on the western fronts like in Germany where these incursions were more regular.

Onto the third issue, the empire was split in two. There was the more prosperous eastern half with their capital in Constantinople and the western half with the capital of Rome which was decidedly more "backwater" to many Romans ironically. Most of the money was found in the east so when the barbarians tried to invade that part of the empire, they paid off the invaders to go bother the western part of the empire instead leading to the aforementioned issues above.

All of these factors and others lead to the fall of Rome, but you can mostly owe this to the fact that the west was left for dead by the eastern half after emperor Theodosius permanently bisected it. And some would argue Rome didn't fall until the 15th century because the Byzantines were just the continuation of the Roman empire under a different name. The power of the empire shifted east and what was left in the west was picked up by other tribes such as the various gothic tribes. I hope that answers your question somewhat. I don't believe it is one event but a culmination of these big events and other smaller ones.