The zipper teeth separated from the fabric zipper tape, but the zipper still works.
First, I removed all the loose threads (I even used tweezers to get the very small pieces). Then, I lined up the fabric with the zipper teeth and stitched between the teeth.I went back and forth from outside to inside and back again.I kept this up across the whole tear, never looping the thread over the top of the teeth – only in between them. I knotted the end and clipped my thread close
It worked! This backpack is usable again!