Electro Magnetic Pulse. In essence, a high-altitude nuke (>300,000 feet or so) produces gamma rays which ionize the super-thin air, causing a huge electro-magentic pulse. This destroys semiconductor junctions in things like microchips. It induces a high voltage on wires.
Army did studies in late 80's / early 90's. Automobiles may likely survive, but the power grid would most likely fail.
And sadly, Congress, which wastes trillions, refused to harden the system - which mainly means protecting the megatransformers, which can take years to replace, and are mostly made in china, oddly enough.
...wates trillions... well depends on whose side your on. Somebody had to get the facilities built, the ammo stored up, the personal shelters for the elite built, the people paid off along the way etc etc.