There are all sorts of fun scenarios involving economic collapse, solar flares., etc - even zombies, but there is one follow on effect that I have wondered about.
There are over 400 nuclear reactors in the world. If we all turn into zombies, what happens to them?
One book that did discuss an eventual follow on effect, though not in a particularly realistic systematic fashion was Summer of the Apocalypse which has a plague causing the collapse, but a much delayed environmental meltdown of the leftover bits causing the existential crises . Possibly McCarthy may have had this being a contributing circumstance to The Road: the novel is certainly grim enough.
Long Blackouts Pose Risk to U.S. Nuclear Reactors
, Associated Press, 29 March 2011 (hat tip: transitionvoice.com)
Long before the nuclear emergency in Japan, U.S. regulators knew that a power failure lasting for days at an American nuclear plant, whatever the cause, could lead to a radioactive leak. Even so, they have only required the nation's 104 nuclear reactors to develop plans for dealing with much shorter blackouts on the assumption that power would be restored quickly.
What this article is worrying about is that if the generator loses its own power, and the grid around it goes down, they do not have independent sources of power to keep the nuclear fuel from melting down. So your various EMP-strike scenarios and Solar Flares could have a catastrophic.
You could make the plants small enough that natural water circulation would be sufficient to keep the pile cool, but this concept did not come into play until the 1980s and most reactors are too large to use this method.
My greater concern is that the default mode of a nuclear reactor is that they meltdown. So if events became chaotic enough that the technicians left the site, there is no east "off" switch to safely turn out the power.