Hi everyone, this happens to me when I copy over a course and forget to change the due date from one semester to another. Canvas automatically assigns 0 to assignments and will not change it back even when you change the due date. 

There is a way to fix this but you have to do this for all assignments. Go to your grades tab. There is a 3 dot icon to the right of each assignment. Click on that and select "Set Default Grade." Leave the box blank, and check the box that says "Over-write already entered grades" and click "Set Default Grade."

If there are real scores from students, and not just "0," you will also over write that score. I hope this helps.