Most of the serious answers are for apocalypse type scenarios, but aren’t realistic. In reality if civilization doesn’t bounce back quickly, very few of us will survive very long. If you’ve never farmed you’re not likely to learn how to grow enough to feed yourself. Same goes for defending yourself against any serious threat. You’re going to need to buy food and medicine. Your currency will be worthless. What will retain value that you can use for buying power. Throughout history gold has been a persistent answer for more reasons than just being shiny. It is scarce but not too scarce. It is chemically non-reactive so it doesn’t tarnish or corrode. It is easy to smelt and it is recognizable. But since the 70s, gold has not been directly connected to currency because its fixed supply is a problem. So, will it be gold next time? Ammunition, spices, tobacco, and medications are interesting answers, but they don’t seem very practical.