I recently gave a quiz for which my students needed more time. I forgot to change the "close the quiz" date, and "open attempts are submitted automatically" was set. Therefore, all of my students' unfinished quizzes were submitted. After searching for "unsubmit quiz" and "reopen quiz" and finding nothing, I came up with the following solution:
1) Change Grade > Attempts allowed to 2
2) Change Question behavior > Each attempt builds on the last to yes
3) Have your students reattempt the quiz. Their new attempt will begin exactly where the first attempt left off.
4) Once they have begun their second attempts, you can delete their first, prematurely closed, attempts.
You will be left with a single attempt per student that began where the original (closed) attempt left off.
This will also work if you need to do it for a subset of students. Instead of changing Grade > Attempts to 2 for everyone, create user overrides for the students who need their attempts reopened.