Charles Stross http://www.antipope.org/charlie writes in his book GlassHouse, page 47:

“We know why the dark age happened. Our ancestors allowed their storage and processing architecture to proliferate uncontrollably, and they tended to throw away old technologies instead of virtualizing them. For reasons of commercial advantage, some of their largest entitites deliberately created incompatible information formats and locked up huge quantities of useful material in them, so that when new architectures replaced old, the data become in accessible.”

“This particularly affected our records of personal and household activities during the latter half of the dark age. Early on, for example, we have a lot of film data captured by amateurs and home enthusiasts. They used a thing called a cine camera, which captured images on a photochemical medium. You could actually decode it with your eyeball. But a third of the way into the dark age, they switched to using magnetic storage tape, which degrades rapidly, then to digital storage, which was EVEN WORSE because for no OBVIOUS reason they encrypted everything. The same sort of thing happened to their audio recordings, and to text. Ironically, we know a lot more about their culture around the beginning of the dark age, around old-style year 1950, than about the end of the dark age, around 2040”