Our Endless Numbered Days by Claire Fuller
1976: Peggy Hillcoat is eight. She spends her summer camping with her father, playing her beloved record of The Railway Children and listening to her mother's grand piano, but her pretty life is about to change.
Her survivalist father, who has been stockpiling provisions for the end which is surely coming soon, takes her from London to a cabin in a remote European forest. There he tells Peggy the rest of the world has disappeared.
Her life is reduced to a piano which makes music but no sound, a forest where all that grows is a means of survival. And a tiny wooden hut that is Everything.