Life is rather uncertain nowadays as anything can happen at any time and we would be caught off guard. Thats also one way to describe the current economy at this stage. With the worlds economy expanding and expanding there have been more crashes as the years go by and they are becoming more frequent and some say at any moment one could happen.
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 dont 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.
I think that if we are talking about an event similar to the one in 2008, maybe slightly more significantly (that is, a global economic crisis), then I'd choose Bitcoin to store my money. It can go down along with everything else, sure, but I am confident that it can recover from the drop, whereas the top fiat currencies might not (they lost too much value through the years already, so a big push might lead to their collapse). Gold might be not a bad idea, but it's just something about me and gold that I don't really value it and feel like having it. If we are talking about a Nuclear War kind of event, I'd buy warm clothes, non-spoiling food, anesthetics, clean water, weapons for protection and some philosophy books like Plato's dialogues not to degradate significantly and not to get crazy (assuming the internet is down).