Students who have withdrawn from the course disappear from the Grades section. Their grades, however, are visible in the Enrollment Statistics through the Classlist. 

Step 1: Go to Tools and click on Classlist 

Step 2: Click on Enrollment Statistics 

You will now be able to see the complete classlist of enrolled students with their grades. To see the grades of a withdrawn student, do the following:  

Step 1: You will find the list with withdrawn students down below titled Withdrawels 

Step 2: Click on the arrow next to the name of the student and click on View Grades. All their grades will then become visible.