With the defeat of Antony and Cleopatra at Actium in 31 BC, Octavian, Caesar's adopted son and heir, was left undisputed ruler of Rome. Proclaimed Augustus four years later, he sought to build a capital worthy of the empire over which it ruled, boasting that he had "found Rome a city of brick and left it a city of marble." And, indeed, the Roman Forum essentially is Augustan: the Temples of Saturn, Concord, Castor and Pollux, Divine Julius, the Basilicas of Julia and Aemilia, the Curia and Rostra--all took their final form during his triumvirate and principate.
In AD 410, six years after the seat of imperial government had been removed to the safety of Ravenna, Rome was sacked by Alaric the Visigoth (one still can see green stains on the marble floor of the Basilica Aemilia where bronze coins had melted). In AD 455, Rome was plundered yet again, this time by Genseric the Vandal, who removed half the gilt-bronze tiles from the roof of the Temple of Jupiter, and then in AD 472 by Ricimer. Only a few years later, in AD 476, the last emperor was deposed and the Roman empire in the West ceased to exist. The temples, basilicas, and other monuments in the Forum eventually were abandoned and despoiled until even their names were forgotten. They were stripped of the lead and metal clamps that had joined them together, the stones reused, and the marble burned for lime to make cement. For hundreds of years, these depredations would continue until, eventually, the site would be known only as Campo Vaccino, the Cow Field.
Not until the nineteenth century would the name of the Forum Romanum be heard again.(source)