Problem

All grades for an assignment, quiz, or discussion have disappeared from the grade book.

Explanation

This issue most commonly occurs when you have adjusted an assignment date for an individual student. Instead of adding a row in the Assign to area, you replaced the Everyone option with the individual's name.  This did not delete the grades completely, it just removed them from the grade book since the assignment appears to only be assigned to one student and not the whole class. 

Solution

This is a common issue with an easy fix. Go back to edit the assignment where you changed the due date for an individual student and instead of replacing the student's name, look below the Available from and select the +Add button. Assign to Everyone (this will change to Everyone Else due to the student name above) add your due and available dates and make sure to save. Now the students' grades will be visible in the grade book again because they are all assigned to the assignment. 

Here is a video under a minute long that walks through the solutions steps.