Here's a wild idea. What if long, long ago somebody already invented "nano technology" and it's now a days known as bacteria and viruses. It was working just peachy keen until one day something went wrong or maybe while everybody was sitting there with their feet up on the desk they forgot the part about to maintain such a society at such a high standard of living, you also have to maintain a very high level of education. Or maybe your "barbaric" neighbors decided to use them against you. Oops.
wasnt that the basis for the sci-fi british tv series ... the excaliber ??? only it was the aliens that infected earth .....