writer's block...

so i'm trying to write a story but i have one big problem:writer's block. basically, the story is set in the near future and 80% of the human race is no longer alive, nature has reclaimed many former cities but a few places remain unchanged. what my problem is is that i don't know why much of the human race is no more. what should i make as the reason for this giant change?

>>7
You're full of shit. I know for a fact that many writers (Stephen King, Charles Dickens etc.) have specifically started novels without knowing how it would end or where it would go. They didn't plan anything out before hand.