On September 4, 476, Odoacer, a Germanic officer serving in the Roman army in Italy, deposed Romulus Augustulus, the last emperor of the western empire. Odoacer had already defeated and killed the boy emperor's father, Orestes, and had taken Ravenna, which had been the western capital since 402.
Romulus was young, perhaps fifteen, and had been put on the throne by his father rather than chosen by the state. Odoacer did not kill him. He sent the imperial regalia back to Constantinople and ruled Italy as king while acknowledging the authority of Zeno, the emperor in the east, which is why the year 476 looks less like a collapse than like a change of who collects the taxes and commands the soldiers.
That detail is why the date is a marker rather than a turning point nobody noticed. The empire in the west had been shrinking for a century, and by the 470s it effectively covered Italy and a strip of southern Gaul. Daily life barely changed there. The Senate went on sitting. Roman law went on being applied. The eastern empire kept calling itself Roman for almost another thousand years.
What ended was the line of western emperors, and with it the fiction that there were two halves of one state. Odoacer's own rule lasted until 493, when Theodoric the Ostrogoth killed him at a banquet and took Ravenna for himself.