A temporary halt of the Bitcoin network doesn't mean it will perish and I do not think Bitcoin enthusiasts will be wiped out during that nuclear war.
This probably will be the case for bitcoin and every altcoin if ever a nuclear war happens. Including of shutting down internet from affected countries, food will be scarce as every daily needs will price hike, also new virus/sickness will grow due from different reasons when there are lots of dead bodies. But this will only happen most probably on the country where the battlefield is.