InfoWorld describes five doomsday scenarios for IT, it includes a coordinated hack attack on the power grid, an EMP attack, a takedown of the Internet, a massive solar flare, and more. You can read it over here. Out of all scenarios described, a massive solar flare is the most devastating one as it could cause a worldwide failure of the electrical power grid and communications systems. A recovery from such a catastrophe wouldn't be easy, experts estimate that in a best-case scenario it would take at least four years to get things back to normal.
How long to recover: Unknown. According to a January 2009 report by the National Academy of Sciences, the effects of a severe geomagnetic storm would be felt for years, most acutely in societies that are the most dependent on technology. The U.S. could take from four to 10 years to bounce back, according to the NAS -- if it bounces back at all.
"It will take a tremendous amount of manpower to clean up the mess," adds Siciliano. "Something that catastrophic, the gas pumps won't be operating, so a guy who's supposed to take a part to repair a facility can't get there because he has no gas. It could literally throw us back to 1840. Suddenly we're a third-world country again."