My first thought when I saw your question was that this particular student was removed from the list of students that were assigned the quiz. If the quiz settings were changed after the student had taken the quiz, that would explain the behavior you see. So the first thing I would do is edit the quiz and on the Details tab confirm that the student is not excluded from the Assign to section of the page. It could be that the student's section is no longer assigned that quiz.
Another option is if your course uses grading periods. I don't have experience using those, but I see on its page in the Canvas Guides that grades can no longer be edited after a grading period closes. That would not explain a grey window but there might be a connection.
How do I use grading periods in a course?
I’m going to mark my response as a solution, but there can be more than one. I hope our colleagues here in the Community can share other options for getting this important information to teachers inside Canvas