Now, imagine a world in which there will no longer be any agriculture, fuel, easy access to clean water, electricity, heating, medicine, health care, communications, government, supply chains, or law enforcement. The food supply that was available at stores and in homes has now run out. Fertilizer is no longer available, contemporary farm machinery that runs on petrol won't last long as the petrol dries up (and when they break down, won't be repairable due to DRM). Wildlife might supply enough calories to feed a small fraction of one percent of all people currently still alive.
This would go on indefinitely.
Preppers look at surviving a nuclear war like it's an adventure where all their Eagle Scout skills will finally be put to use. It's really just slow, heartbreaking death from starvation and disease, with horrible violence from desperate people and regional warlords mixed in. Think "The Road", without the extreme environmental destruction.
Note that this wouldn't be everywhere, just in targeted countries. US, Canada, NATO and NATO leaning Europe, Japan, both Koreas, China, Russia. Those not targeted would be looking at this whole thing kinda like insert "it's free real estate" meme here
And if nuclear winter is a real thing, which most contemporary modelling suggests it is (though this is not settled science and some modelling suggests it isn't), likely there won't be an environment conducive to growing crops anywhere in the world for at least 5 years. A lifetime Prepper Magazine subscription won't help you there.