That is the part where federal government gets involved.
First they will need to suspend the enforcement of all sales tax on an emergency basis. Secondly, they need to permit and facilitate the temporary use of IOU, written credit, and verbal bartering. They might just issue bulletins and distribute documents that give example of the proper way to make written agreements for between suppliers/vendors and consumers. This will get abused by many vendors but they can take those documents back to the fed for conversion to real or digital cash.
What would we barter, I wonder? We are no longer farmers, mechanics, or wheelwrights; we hardly produce anything nowadays. Our service, without a digital infastructure is not worth much either. I suppose the wages of the truly unskilled service worker in a non-digital world would be governed by Ricardo’s Iron Law of Wages, that being the minimum necessary to prevent starvation.