Oh thats easy. Roving bands of tribal/ferals searching for food and trying to survive. Slavery. Remanants of America’s military transitioned into Merc groups. Depopulated cities crumbled into ruin. Technology returned to the middle ages for the most part except for occasional places that subsist on scavenging parts that still work.
Just play the Fallout video games. Todd Howard was a prophet, though I doubt it was his intention.
All this, of course, with a much reduced population.
There are a number of factors that could bring orderly & constructive social interaction to a screeching halt, virtually over-night. What is being overlooked are contextual phenomena that will compound the rush to chaos. (For example, how would urban supermarkets keep the shelves very far from empty, in a currency collapse such as the Germans & Hungarians "enjoyed" in 1922 & 1923. Or consider the less dramatic, but at some point immediate effect of a distant Federal Bureaucracy, replacing the Hippocratic Oath as the foundation for health care services.)
Have Fallout 3 on PS3. Excellent game!!