The late and deeply lamented Charles Sheffield wrote a pretty good book about the entire planet being whacked by just such an intense electomagnetic wave burst. It came from (iirc) a star whose nova(?) was aimed in our direction.

I read it several years ago so I'm fuzzy on the details. But what I remember is his interesting and detailed extrapolations. Older cars were suddenly very valuable since they had no computerized parts. Strange breakages happened like fridges, furnaces and gas logs--who knew how many things had delicate electronics built in?