A 24 hour penalty would be assessed on their departure time for the following leg; it probably would lead to an eiimination, depending on what is planned (a long plane trip with an HOO could well reduce the gap significantly.)
In the early seasons we did see teams fall behind by 24 hours on an NEL, and they continued Racing. In TAR 1, Margarita and Frank had a 24 hour lead headed to India, and by the end of the next leg the other teams had caught up in India; In TAR 11, one team was a day ahead in arriving in Tanzania, due to flight scheduling issues in an heavy travel period in Africa, but an HOO allowed other teams to catch up; and in TAR 1 beginning in Thailand, two teams fell 24 hours behind the lead teams, and the last team was eliminated in Beijing, and the other team never caught up before the end of the Race, they had just started the final leg when the winning team reached the finish line.
One thing is clear the show does not switch NELs around because a team fell far behind, or quit the Race, or anything else. Its a governmental rule dealing with contests on television that prevents it, and the producers literally have to have put the plans (including alternative legs and so forth) in an sealed affidivit before filming starts. (That may have changed at some point, but it was clearly the case during the early seasons.)