I'm fascinated by the idea of a massive world-wide loss of knowledge. I been rereading some of my favorite childhood books--The Tripod series John Christopher. In the books watches and clocks are rare, train cars are pulled by horses, and large cities are completely abandoned. In The Gunslinger the characters also come across abandoned cities, are baffled by the cars they find, and have somehow lost all technology.
I wonder if this could really happen to us. In some ways, it already has. We have lost musical instruments. We have lost the knowledge of medicine men. Heck, we speculate how the pyramids were built. Will people one day come across our old cell phones and mp3 players and wonder what they were?