The sun didn’t come up.

(from Zoetrope All-Story,Vol. 11, number 2)

And in these memories, of course, they would always later place one more object in the scene. Ridiculous to have thought of then; almost like remembering that your lungs filled and emptied themselves of air each moment, or that your heart dutifully pumped its ration of blood. Glowing dimly in every memory: the sun.

You could call the two characters at the center of this story star-crossed lovers, in that the sun’s sudden disappearance is their primary antagonist. Why has the world been plunged into darkness? We don’t know and, frankly, I would only want to know if this story was merely the opening chapter in a novel, which maybe it should be. But as it stands, Helen and Louise don’t know they’re bit players in a post-apocalyptic adventure story. Some hero scientist or hero intern or something is working on the problem. These two are only heroes to each other. They just want to ride out into the darkness together. This story is magnificent.
You can read the beginning of it here.
This poem by Byron, also called “Darkness” and quoted in the story, seems to have been the primary inspiration.