The problem is money and credit, or more specifically, the lack of both. In a cashless society, we depend on ATMs and credit transactions to purchase almost everything. Truckers and suppliers depend on credit to purchase fuel and inventory. When the grid goes down for any length of time, we lose our money and there is no credit to purchase anything. No supplies, no food, no gas, no nothing is delivered, and ever if it were, there would be no cash to purchase it.
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.
Industrial feudalism to flourish? Those knowledgeable in the arts of a technological society, seek patronage with those with the ability to organize?