This is a theme that resonates with our own history, as many cultures in Medieval and later times viewed ancient Greece and Rome as just such a lost Golden Age. In a time when simple sanitation was nearly non-existent, the idea of a civilization advanced enough to create aqueducts sufficient to see to the needs of cities of hundreds of thousands of people, or the creation of enormous stone monuments or roads that endured for a millennium after their builders were dead, was a powerful attractor.
In a fantasy campaign, this can be handled in a variety of different ways. Political leaders seek to recapture the lands and powers of their predecessors. This is a theme that looms large in Medieval and Renaissance history, when emperors such as Charles the Great sought to be “the next Roman Emperor”, not to mention the Popes seeking to fulfill that very same role, and the establishment of the “Holy Roman Empire” which, entered as it was in Germany and only occasionally in possession of the city of Rome itself within its territories, demonstrated the pull of the ancients even in circumstances that differed greatly from the historical model.
The idea of recapturing lost knowledge and technology of the ancients is also a powerful theme upon which the game master can draw. As mentioned above, the creation of the more powerful magical items is an art that is more properly left to ancient times, and the creation of artifacts and relics doubly so. Still, the hunt for the secrets of such manufacture can be an inspiring theme for a campaign, whether driven by an individual or an entire organization dedicated to such scholarly pursuits with real-world implications.
It is possible to take this even further and postulate that the ancient world was one in which technology as we know it today was known and put into everyday use. Indeed, advanced weapons, robots, computers, and even spaceships would be seen as merely another sort of magic by the inhabitants of a world where the art of technology is the stuff of legend. This need not lead the campaign into the realm of the post-Apocalyptic, which is in and of itself a different genre of role-playing game, but if the ancient world is placed sufficiently back in the distant past, beyond history and legend and into myth, the discovery of a rare artifact from that dimly-remembered time will seem even more magical.
(This is an expanded section of the Adventures Dark and Deep Game Masters Toolkit, which will be included in playtest version 1.1, hopefully to be released soon.)