Normal commerce ceases, but hastily constructed emergency systems provide society with basic needs. FEMA convenes emergency councils to direct local areas. While it is a physical disaster like a hurricane or flood, everyone understands what is needed and cooperates to reconstruct and begin anew. In addition, some telephone and federal government systems, including the IRS, can't function. The public panics, besieging government offices.
Bank runs develop and many banks fail.
Financial markets collapse. Large cities erupt in rioting and looting. Some local governments continue to function, but many government units and businesses can't. A new dark age lasts for several years. We'll need to keep an eye on this issue to better determine its effects and how to cope with it.