There are several reasons a zipper won't zip. The teeth may not line up or one of the teeth may be missing. The zipper may be stuck. The zipper pull may be missing. All of these can be fixed in a short time with a few simple tools. A zipper with teeth missing will have to be replaced, though.
Using a bar of soap, a wax candle or a graphite pencil, rub all of the zipper teeth on both sides.
Try the zipper.
If it works, you're good to go.
If it doesn't work, repeat with the item you originally used or switch to another one.
Using a seam ripper, remove the zipper.
Measure the zipper and buy another one of the same type.
Pin the zipper in place.
Sew the zipper in place.
Try it out to make sure it works.
Purchase a zipper repair kit.
Find a pull that fits your zipper.
Attach the zipper pull to the loop on the slider.
Try the zipper to make sure it works.
Soap or wax works best on plastic zippers.