Zipper is stuck
This link should help:
Nati I hope this helps you out
If the teeth are just stuck and not broken, you should be able to loosen it by rubbing pencil graphite or candle wax on the teeth. Be careful not to get any on the cloth and ruin your clothes.
Gently work it up and down the zipper teeth. Look to see if fabric is caught. With a little patience you should be able to work it loose. If the zipper is misaligned, gently work it down until you can slide it off. Then realign the zipper.
You can remove the clip at the bottom, and slide the zipper back on. Then see the zipper where the clip was remove.
Look carefully at the teeth on both sides of the zip to see if any are bent, broken, or if any may have caught a piece of thread. Also check the underside of the slider to see if it has been bent or broken. Is the stitching that is holding the zipper on to the garment all still intact? Sometimes even a few loose stitches can cause a zipper to no longer want to lie flat.
This article might help
This video may help.
Have you tried using a wax candle?
You need to gently work the zipper back down from where you started. There is probably some fabric at the back that has been caught up with the zipper so work the zipper tab gently until this loosens. A good silicone spray helps to keep zippers sliding nicely and is usually does not stain your fabric because it is clear.