I must admit, my particular theory is influenced by a couple of books. If you care to read a bit:
The novel Eve, (based on an online game that I've never played, but the book was very good), by Tony Gonzales. In this book, human society reached a high technological level- much higher than what we have today. They were fully space-fairing people who began to colonize nearby worlds. Something strange to them is that there was no fossil record of their evolution, like there was for the other creatures. It remained a mystery until, in a forbidden sector of space, a derelict vessel was discovered, and using some rate of decay and erosion by cosmic rays dating method, they determined that the vessel was close to a million years old, (or really really old anyway). I don't recall the particulars of the book as I read it several years ago, but they discovered that their race came from a planet many light-years away called Earth. Essentially the colonial society rose and fell many times on the new planet, going through similar phases of societal evolution across a huge gulf of time that we did here on Earth. They went through a very long "dark ages" type period before a renaissance eventually led to the redevelopment of high technology.
David Weber's Safehold series also influences my idea somewhat. In this series, Earth was attacked by an alien race far in advance to our own, drawn to us by our advancing technology. The human race was obliterated, but a colony ship managed to escape. The ship contained the genetic material for a population large enough to start again on a new world, Safehold, which included humans along with plants and animals from Earth. What's interesting is that technology would be forcibly repressed on the new world by religious beliefs in order to avoid the fate that befell Earth. This enforcement would be carried out by a handful of architects- extremely long lived humans who would use technology to convince the colonists of their divinity. Advanced technology would be indistinguishable from magic to the simple colonists. These human deities contrived a series of conscriptions, (like commandments), that would prevent the development of technology beyond that of simple lever and water-driven technologies. They actually had orbiting satellites that could inflict "divine punishment" to any who violated these proscriptions. The architects themselves eventually split into two faction, and to be people of Safehold, the gods were at war. Parallels could be drawn between these architects and the Aedra and Daedra of The Elderscrolls universe.