An EMP attack would take place with a high altitude detonation of a relatively small nuclear device, close to the size the North Koreans are testing. It would generate gamma radiation that would likely take out the power grids and pretty much anything with integrated circuitry.
It could literally takes us back to the 1800's in a few seconds. None of the systems that run our country i.e. power, water, hospitals, mass transit, are hardened in a way that would let them survive such an attack.
There is currently a bunch of stimulus dollars marked for upgrading the power grid and a call was made to use that to include hardening to the upgrade.