@K_G This is a common issue.  You just need to click the little "+ Add" button at the bottom of the "Assign to" box to add a new Assign to box.  Then put the "Everyone" back (it will most likely say "Everyone Else" as Canvas realizes you are assigning it to everyone but the student you singled out).  This will reassign it back to your class.

You want to use the "+ Add" when you are giving a student a different due date and not switch the current assign to box as that is unassigning it for everyone else.  Once you re-assign it back to the class, all the submissions and grades will return.

Hope this helps!

-Nick