Several years ago I posted a summary on a Q thread of an NSC study directed by President Kennedy in 1961 to war-game a scenario in which the Soviet Union executed a successful nuclear decapitation of the federal government.
The study stated that in the absence of civilian authority, the military would take over government of the country until civilian control could be reestablished. The military would control all means of communication, transportation, and state and local governance. Chinese-style work brigades would be organized by the military to draft citizens into assisting, and providing labor to, the military. Eisenhower had already put together executive orders along these lines, and Kennedy and later administrations kept these orders up to date.
The NSC study also stated that the House of Representatives could only be filled by special elections conducted by the states. Governors, for the most part, could fill the Senate under the terms of the 17th Amendment. The Electoral College chosen in the most recent presidential election would be the only body legally permitted to elect the president and vice president.
The results of ths NSC study are still valid and in effect.
That's reassuring.