If the zipper comes out, this will be easier.
First, you take out the broken suture with a needle.
Second, zip up. Turn the reverse side out.
Then sew it with sewing machine or hand needle, and sew it horizontally at the tail of the zipper to prevent it from falling off again.
Finally, turn it over and it will be OK.
If the zipper is broken, buy a zipper of the same model and color (of course, the color can be different).
Just take it apart and sew it according to the above steps.