To get an idea of the scale of the problem you have, you can do
SELECT * FROM mdl_quiz WHERE grade = 0 AND sumgrades > 0
(using a tool like phpMyAdmin). If there are not too many, you can fix them by hand.
If there are lots, then you may choose to take the risky option, and edit the numberes in the mdl_quiz.grade column directly in the database, which will be quicker.