In the mass-dissapearance scenario from my previous question, what happens legally if both houses of Congress are in session and a majority of both houses disappear? Is Congress unable to do business because there is no longer a quorum? Or does the quorum become half of the remaining members?
Likely nothing. Both houses cannot enact legislation without meeting a quorum, which is a more the majority number of members present, so any voting would be suspended pending seats being filled by emergency election or appointment.
Typically, in the U.S., when a Rep seat is unexpectedly vacated emergency elections are called, while the Senate would have vacancies filled by the state's governors until the end of the term as determined by the seat's election schedule.
Given that Representative terms are 2 years in duration, Congress has staffers who are there specifically to help freshman Congress members get up to speed on the nuance of Congress, as well as staffers.
If you want to look into a series that does look directly at a sudden loss of Elected Federal officials in the United States, the tv show "Designated Survivor" has this as part of the central plot, with the main character being the Secretary of Housing and Urban Development who was named the designated survivor for the State of the Union (during which, the capitol building was bombed in a terrorist attack) and finds himself suddenly the President of the United States. One early episode deals with the plans for the reconstitution of Congress and another episode deals with the Reconstitution of SCOTUS. While I'm not sure if it's an actual practice, at least one person from Congress is revealed to be the Congressional equivalent of a designated survivor, but I'm not aware if the role exists (Since leadership is voted on in each new session and sessions are two years.).
It's not always the most accurate, and at times tends to be a bit overly naive about U.S. politics but does deal with the sudden emergency decently.