One day, all the computers stopped working. Everything stopped, and nobody knew what to do.
A simple premise, and one which is an increasingly likely scenario for a "Black Swan" event. The basic idea is this: If a more malignant version of software like the Stuxnet "worm" (the computer virus that shut down almost 60% of Iran's computer network last year, and which is now widely available for dissection on the internet) were to be deployed against the USA, there would be absolutely nothing we could do to defend our infrastructure - particularly our electrical power grid.
Stuxnet proved that a computer virus can affect the physical world by causing pipeline overloads, dam breaches, power plant explosions, and other damage. However, having been (ostensibly) designed by American/Israeli intelligence agents, it contained certain self-limiting instructions that prevented widespread death and destruction. It would be wise to suspect that terrorists of any persuasion would not extend the same courtesy to our civilian population.
Consider, for a moment, how dependent we are on computers, particularly for information. Without computers, there are no radio or television broadcasts, no newspapers, no telephone calls (including via landline). Then, of course, there are the implications of massive and long-term electrical outages.
The fact is, terrorists - or hostile nations - don't need an EMP or a battery of nukes to wipe out our civilization as we know it: all they need are a few computer geeks and an unknown amount of time.
Perhaps I'm tilting at windmills here, but it seems to me that we should start reconsidering some of that analog, mechanical technology that our governments and businesses were so eager to replace with "more efficient" computer technology.
In the words of a character in the 1981 Theodore Roszak novel Bugs, "I wonder if anybody remembers where we buried all the slide rules and adding machines?"