If you have an interest in the potential disaster an EMP can cause you should read some of the books recommended by Freepers in this and similar, threads.
Briefly, it is theorized that an EMP would burn almost every computer and computer controlled device within its range.
Just about the entire electric power and distribution grid, communications system, banking, transportation, food supply and distribution, automobile, elevator, cash register, etc., etc., is computer controlled.
Once a computer or computer chip has been fried by an EMP it is gone. It won't fix itself once the EMP passes.
That means millions of computers controlling every aspect of the electric grid, communications, banking, finance, transportation and industry would be knocked out for good until replaced or repaired.
The EMP would likely fry the small amount of spare parts available too. Most computers are foreign built or assembled here using mostly foreign parts. Once the grid is down and you have no communications or transportation it would be a long time, if ever, before the necessary spares could be located or manufactured and transported to the places they are needed.
And then there is the belief that a powerful EMP could also short circuit larger and more robust components of the grid - like the transformers you see on power poles and alongside streets. There are millions and millions of them nationwide. It would probably take 20 years or more to manufacture replacements for all of them. IF you had the manufacturing capability, which we don't.
And it is a Catch-22 situation - once the computer controls and transformers are burned out there won't be any factories at all operating, no way to order parts, no way to secure raw materials, no way to pay for anything, no way to transport anything, etc.
Would it fry things that weren’t plugged in and running?