A total nuclear war happens between two superpowers, and destroys most of the northern hemisphere and parts of Oceania and South America. 42,000 nukes are used, and hit dozens of cities and cause mass destruction. Humanity survives, though it was a challenge. My story takes place mostly in the southeast, Midwest and southwest regions of the United States. Most people live in small farming communities, with technology equal to 17th century tech. They mostly trade with other nearby cities, though it is a challenge, as wild animals and occasionally marauders attack settlements. The fastest method of transportation is a horse, and no one has seen a car move in generations.
Some settlements have banded together into United City states, but that is fairly rare. Though modern medicine doesn’t exist, germ theory still does, so it’s not as bad as it was back in the Middle Ages, but not as good as today. The human population is only about 2 billion; in the United States 200 million people live. Most people are subsistence farmers, but large plantations do exist in states like Alabama and Georgia, and use slave labor (not specifically of African Americans) to do the work. It’s been about twenty human generations, specifically 497 years.
My question is, what would be the main challenges of rebuilding modern or close to modern civilization, and would they even be able to do it?