Simple fixes to most zipper problems include rubbing graphite on a stuck zipper, using pliers to close the slider when the teeth separate or adding a new zipper slider onto the teeth. The correct fix for a zipper depends on the type of zipper and problem.
When a zipper is stuck, rub graphite from a lead pencil on the zipper's teeth. The graphite lubricates the zipper teeth and makes it easier to pull. Bar soap or lip balm both have the same effect.
If your zipper has separated, use pliers to squeeze zipper teeth back into place to make it work again. However, you may need to replace the zipper slider if it is old. To do this, squeeze it off the zipper with a pair of pliers, and zip the new slider onto the teeth.