It would take an absolutely apocalyptic event (or more likely, series of events) to make the human race go extinct but it wouldn't take much more than a prolonged, widespread power failure to push civil society to collapse.
Dies the Fire is a first of a series of novels about a global power failure due to an alteration in physics by an unknown cause. Most humans starve to death, but some in the right locations resort to farming by hand, hunting and fishing. Others set up feudal kingdoms.
