Most of us have experienced the aggravation of a zipper just abandoning its track. If you can't easily get it back on and don't want to bother with replacing the whole zipper, here's a quick fix.
Essentially, you'll cut a slit on the zipper tape that's not connected to the zipper pull (between the teeth and below the pull). Then slide the pull over the open slit and all the way down. Stitch the opening you just made so you can pull the zipper back up and down. Here's a visual:
For more photos and step-by-step directions, see the post on the Make It or Fix It Yourself! blog. Or see our guide for more zipper problems and their solutions.
How to Quickly Repair a Broken Zipper [Make It or Fix It Yourself!]