As a child of the 80s I was scared shitless by a drama called Threads simulating an attack on Sheffield. It's the proper stuff of nightmares.
The only people who have nuclear weapons are a select group of countries. These weapons work in a similar way to the nuclear weapons used on Japan, but orders of magnitude more powerful.
The principle of mutually assured destruction still stands I think. They are last gasp apocalyptic weapons with very secure command and control systems.
The real worry is material be used to make a so called dirty bomb, with radioactive materials placed around an explosive conventional core. A very crude device but would cause much terror. Even these wouldn't cause a nuclear winter though.
There was something on C4 a few years back about a destructive solar flare wiping out electronic and power systems. Within a day food supply chains had collapsed and people were killing for supplies. A major solar event is long overdue, think the last one was in victorian times.
Also, the Earth's magnetic field has a habit of reversing every few million years. Again the effects would be as bad as the solar flares.....
Then there's asteroids.........
Mind you there have been massive extinction events throughout history, and life has survived. These may have been caused by seismic activity, climate change, the rags and dippers.......
But the point is human species have only been on the planet a few million years, modern humans only 100 - 250 000 years. Proper Johnnie come latelys. Perhaps we'll make first contact with an intelligent peaceful alien species and see the error of our ways!