The year is 2020. A massive arctic high sits over North America on a very cold January day. An Al Quaeda operative sends a command from his PC to a botnet which activates multiple zero-day "sleeper" trojans that have been waiting for the command. PC's, printers, and various other machinery in offices and electric power plants and water pumping stations and telephone offices fail.
A second command is sent that hijacks satellite downlinks for GM Onstar and similar systems. They can shut down the car via satellite if it's reported stolen. In the middle of rush hour, traffic grinds to a halt, as the shutdown code is activated en masse. You have millions of motorists stuck on the roads in bitterly freezing weather. +1 if the system can be programmed to lock all car doors, and trap the motorists inside.
Another command is sent out that cranks "internet of things" furnaces and stoves to max power and locks them there. Fires break out all over. Fire departments are unable to respond. Even if their trucks don't have Onstar tech, the roads are so clogged with stalled cars that they can't get to any fires.
Civilization breaks down as distribution chains collapse due to non-functioning equipment. Millions die of cold and starvation in the following weeks. Martial law is declared. Somewhere, in the middle-East, a bunch of Mullahs are laughing their butts off.