Poul Anderson wrote half a dozen post-nuclear war scenarios. More recent thinking on an all-out nuclear exchange is that it would cause a Nuclear Winter and that no life would survive. But there are other ways to imagine the end of civilization. Anderson also has an Ice Age and an interstellar Long Night. Other authors have comets, floods, droughts, diseases, blindness plus Triffids etc. SM Stirling's Change is ingenious: technology stops working and that suffices as a premise although we suspect that malign aliens are behind it.
The moral of all these stories is that, physically, our continued existence is both vulnerable and precarious.