A vast majority of modern people would die a helpless death relived of their modern inconveniences. You don't get time to "learn" how to farm when you don't have any food.
And farming is only 1 or 2 generations removed. Give it a few more turns, and one can easily see the world in the story becoming a reality.
I've lived on a farm for a couple of years (in Ontario), and it is simply amazing how much hard work goes in to making a piece of land productive, even with modern tools.
In aerodynamics as applied to jet aircraft they have something called the coffin corner, a speed so close to the limits of the flight envelope in all directions that a small change in velocity will make you either stall or break up.
It's possible that there is such a thing as 'societies coffin corner', a speed of development so great that if it gets exceeded by a little or drops for a short time that we'll literally crash.
Vernor Vinge's stories have addressed a very similar idea. "A Deepness in the Sky" has significant interludes on this topic, though it's not the main focus of the work.
There might well be things that people used to do that we don't even know they did. Obviously I can't give specific examples, but maybe something like a particular way of hunting animals? We wouldn't even know that we lost this technique because its so far removed from anything we need to do.
