The first event that contributed to the fall of the Roman Empire was the invasion by Barbarian tribes. The fall of continuous military losses against the outside forces pinned the collapse of Western Rome. For centuries, German tribes have been in contact with Rome, but the borders of the Empire were encroached by the Goths that were also known as the barbarian groups.
Further in the late 4th century, a Germanic uprising was weathered by the Romans, but the city of Rome was sacked successfully by the Visigoth King Alaric in 410. The next several decades were spent by the Empire under the continuous threat before in 455, the eternal city was again raided by the Vandals. Finally a revolt was staged by Odoacer – the Germanic leader in 417 and Augustulus – the Emperor was deposed. From then no post was never ruled again in Italy by any Roman emperor. Deathblow was suffered by the Western Empire.
Another major reason was overdependence on slave labor and economic issues. A lot of financial issues were faced by Rome along with under attacks from the outside forces. Imperial coffers were lightened significantly by continuous overspending and wars. Further the gap between poor and rich was widened by oppressive inflation and taxation. Independent fiefdoms were set up by various members of the wealthy class who also fled to the countryside to avoid the taxman. At the similar time, labor deficit hit the empire.
The economy of Rome was highly dependent upon slaves including craftsmen. A fresh influx was provided of conquered individuals placed to work traditionally by its military. But when supply of war treasures and the slaves began to fade up, an expansion hit in the 2nd century. In the 5th century, a further blow came when the trade of the empire got disrupted by North Africa being claimed by the Vandals.