I decided to contribute my 3 cents (inflation has raised its value) for
the first time because of something I think many people are forgetting. In
Compensation Management (Another course I teach), we often talk of
individual equity" What about the other students? In effect, by giving Johnny
 (the person
whose exam was lost) an automatic A, aren't you  penalizing the other
students. Unfortunasely, I don't know that I have the perfect solution.
Also, it might depend on when you lost the exam/quiz (was it the day the
students took it or 3 days later). As of now, my thinking seems to dictate
that you give the student the option: Take the average of other grades on
similar quizzzes or re-taking this quiz.