Student's quiz attempt got submitted before student completed the quiz
The default quiz setting is for "open attempts to submit automatically". Therefore in a timed quiz, a student's attempt will get submitted properly even if they forget to click submit. On occasion, students may have an issue and their attempt gets submitted before they have completed. Here is how to allow for another attempt without have to merge the two attempts or sort out where the student was in the test.
1) Go to your Quiz Settings
2) Change Grade > Attempts allowed to 2
3) Change Question behavior > Each attempt builds on the last to yes
4) Have your students reattempt the quiz. Their new attempt will begin exactly where the first attempt left off.
5) Once they have begun their second attempts, you can delete their first, prematurely closed, attempts in your quiz results. Be careful to make sure you delete the correct attempt!
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.