Here is a conventional, maybe mundane, idea. One of the planet's super volcanoes (there are 6 according to a rather cursory search) erupts and takes out almost all of earth's population. According to another mundane search, the Toba eruption somewhere around 70,000 years ago was a near extinction event in that it left, according to DNA analysis, roughly 10,000 humans alive. If true, folks, we as a species were almost a footnote in the planet's history.
Anyhow, if severe enough, such an eruption would screw up the food chain, i.e. agriculture, for decades. As such, survivors would have a real challenge trying to survive.
Now, since I'm rather creatively challenged, I offer my services as a research flunkie to any creative author willing to wrap his creative consciousness around such an idea. Any such brave soul would have to put up with me as an editor in his creative endeavor.
I admire how Al Steiner took an asteroid strike and turned it into a creative tour-de-force (Aftermath). The challenge would be to handle the dissolution of society in a better and more logical manner than Steiner did. Since it would have to deal with far smaller populations than existed in Aftermath, I believe it permits far more number of options for a story.