So you are left with a situation in which the money you currently own, fiat money: the dollar bills & coins, will become worthless and you cannot even buy anything you need to survive on a day to day basis. In this situation, people will have to resort to a system of bartering like in the stone ages. I guess if you have a lot of stuff with value you can trade that would be great, but a majority of people don’t have such a stash. Things can get ugly real fast.
Therefore, in such a doomsday situation you have to come up with a situation in which you can trade with people in order to survive. There are two solutions I can come up with, either using precious metals such as Gold & Silver or using Cryptocurrency. Considering that fiat currency is useless, Cryptocurrency might raise in value so that it can be useful as a means of trade as long as the Internet still exists which will most likely be the case.