When a nuclear device is exploded above the earth it ends up creating an electromagnetic pulse (EMP). In less than a billionth of a second this EMP will make circuits fry, power lines overload, and the electric grids collapse. Everything with a microchip stops working—including your car, cell phone, and computer. A lot of hospital equipment will go down including in the operating room. Some of the equipment on airplanes would be impacted.
Most people wouldn’t be physically harmed (except for people wearing pacemakers), as there generally is no radiation to worry about when a missile explodes in space.
I bring this up to let people know the danger nuclear weapons can bring upon a country being attacked. Everyone should care about the U.S.'s approach towards reducing the threats. Obama's strategy was laissez-faire as seen with the deplorable Iran Nuclear Deal.