Hi @NastaranKazemei 

If you need to re-open an assignment for just one student, make sure that you do it by clicking "+Add" and assigning it to that student with different dates.  When you to this, the existing assignment that says "Assigned to: Everyone" will change to "Everyone else".  Do NOT just replace "Everyone" with the student's name, or you have un-assigned all the other students from the assignment.

Fortunately, this is fairly easy to fix.  Just go back to the assignment, click on "+Add" at the bottom of the assignment to add another assignment , and set "Assign to: " to "Everyone else", and re-enter the original assignment dates.

There is a description at https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-assign-an-assignment-to-an-individual-s...