There's plenty of information out there about CMEs. No need for me to go into great detail. Basically, the sun burps and sends a huge energetic shock wave to planet earth. That little electro-magnetic kerfuffle knocks out just about everything electronic. Grids go down, car electronics are fried, computers become awkward bricks.
Just about everything that makes 21st century living possible is shut down. Take away city services and cities are suddenly dropped back into the Middle Ages. That is, with way more people and without the infrastructure that allowed Medieval cities to function. At least back then they had some sort of rudimentary methods of dealing with water, sewage, and food supplies. Modern cities depend on modern technology to function. Take away the technology, a city no longer functions.
Oh sure, it might be possible to run some old style diesel engines and other bits and pieces of modern tech. Big deal. Say you get an old diesel car running. What will you do when the diesel runs out? Some military tech might function for some time, but support systems probably will fail.
Not scared enough? There is one modern class of technology that will function for a long time. It has plenty of spare parts and supplies. CMEs will not affect firearms at all. It could mess up things like fancy electronic scopes, but guns themselves will function just fine. Picture that, absolutely everything we count on to live won't work. The only things that will work are guns. Not a pretty picture.
I, for one, would not want to be in a city should that ever happen. You might stand a chance way out in the country -if you have food, a woodstove, a spring for water, and guns of your own. It might help to be snowed in so no one can get to you. However, in the spring, you might get a visit from any hard cases who survived the chaos of the cities. If survival of the fittest means anything, those few survivors are going to be some tough mean bastards.
By the way, the sun is becoming more active once more. For extra credit Google "Carrington event of 1859." It's only a matter of time before something like that, or worse, happens again. We've become a lot more electricity reliant since 1859.
1 hour ago