Gradebook does not include ungraded items in the course grade calculation. This could result in a higher course grade average for a student if they have incomplete items. To include a Gradebook item in the course grade calculation, enter a grade of zero or higher.
To enter zeros for ALL Gradebook items that have not been graded, use Set Zero Score for Empty Cells. You can not this action once done, though can you still manually adjust scores in each cell if you discover that you've made an error.
See How do I set empty cells to zero for a single grade item? to take this step for just one item in the Gradebook.
Select Gradebook from the Tool Menu of your site.
Caution: You cannot undo this action, so be certain that you wish to assign zeros before continuing!