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?
Rule #1: "Writer's block" is bullshit. Real writers write every day. No excuses.
Rule #2: If you find yourself unsure about a certain justification or direction, and being well aware that you have to write today anyway (see rule #1), then just write on a topic in that story's universe as it pleases you. How do people farm? Write about that. How do they trade? Write about that. How does nature creep back into a large city, bit by bit? Write about that.
If you follow these rules, eventually you come upon a theme for your story. Literally, you'll be hammering away at some sort of encyclopedic entry about how the tsunami from the Canary Islands happened in the year 2237, obliterating the Eastern Seaboard of the USA ... and suddenly it'll strike you that Humankkind could have faced a tectonic extinction event. The Sumatra supervolcano could have let loose. The air became too clouded with thousands of cubic miles of ash. Photosynthesis nearly failed. Man lost almost all of his animals and all his crops. Man fed on man. Etc.