It has never happened, so the best we can do is speculate with what we know. The US and Russia did conduct some atmospheric nuclear detonations that unintentionally created EMP's. And determined yes it would be a bad day.
"The critical national infrastructure in the United States faces a present and continuing existential threat from combined-arms warfare, including cyber and manmade electromagnetic pulse (EMP) attack, as well as from natural EMP from a solar superstorm. During the Cold War, the U.S. was primarily concerned about an EMP attack generated by a high-altitude nuclear weapon as a tactic by which the Soviet Union could suppress the U.S. national command authority and the ability to respond to a nuclear attack—and thus negate the deterrence value of assured nuclear retaliation. Within the last decade, newly armed adversaries, including North Korea, have been developing the ability and threatening to carry out an EMP attack against the United States. Such an attack would give countries that have only a small number of nuclear weapons the ability to cause widespread, long-lasting damage to critical national infrastructures, to the United States itself as a viable country, and to the survival of a majority of its population." [Assessing the Threat from Electromagnetic Pulse (EMP), Executive Report, July 2017]
From https://www.cisa.gov/sites/default/files/publications/19_030...