I think cash would still work, unless people stop trusting fiat at all due to the likely extremely high sudden price changes (black markets & barter) and due to the loss of purchasing power.
Of course cash will still work but the problem is that you should have had cash already before the "event" otherwise considering the fact that almost everyone stores their fiat in banks and the banking system can also be affected by the same "event" we can assume people's access to their fiat is cut off.
I think it was Bangladesh a couple of years ago when people formed long queues in front of banks to get their cash out and the banks didn't allow them. Life practically stopped and it showed people don't really store cash.