The Fall of the Empire was a gradual process. The Romans did not wake up one day to find their Empire gone!
By AD369 the Empire was beginning to crumble for the following reasons:
The Government was running out of money.
The people had to pay very high taxes - up to a third of their money.
The rich were given grants of money and land which made them richer while the poor got poorer.
There was not enough money to pay for the army.
Barbarians from Germany called vandals were conquering parts of the Empire and there were not enough soldiers to fight back.
Although the outer edges of the Empire were well defended, there was no defence with in the Empire. This meant that once barbarians had broken through there was nothing to stop them marching to Rome.
The Roman network of roads allowed invaders an easy route to Rome.
No one had decided on a good way to choose an Emperor,. This meant that any general could march into Rome, kill the Emperor and make himself the next Emperor. In 73 years there were 23 Emperors and 20 of them were murdered.