Reason
If the grades for all other students have disappeared from the gradebook after you have provided one student a new due date, this is due to replacing "Everyone" in the due date area with a single student's name.
Solution
Edit the graded item
Go to the graded item and click Edit
Click +Add
Scroll down to the Due Date area and click on +Add. A new Assign To/Due Date area will appear.
Select Everyone in the new area
In the new Assign to area, select Everyone and Add a due date.
Save the changes
Click Save.
The grades will now reappear in the gradebook.
Prevention
To prevent grades disappearing from the gradebook in the future:
Add a new due for the student
Click the +Add button and add a new due date for a student, rather than replacing "Everyone" with the student's name.