Tomorrow all nine countries possessing nuclear weapons launch them in an act of mutually assured destruction. What would the next 100 years look like?

I'm reading a lot of conflicting information online. I believe we are estimated to have around 10,000 nuclear weapons today, with around 50,000 during the Cold War. How long would the nuclear winter survive? Would we able to start up civilisation again or would we all be restrained to bunkers under the ground? Is the threat overblown or is it life-ending? Thank you.