To fix a sticking zipper, you can rub a candle along the teeth and then run the zipper baqck and forth this should alleviate the problem area. If the zipper should come totally apart, take a deep breath. Grab some pliers, needle and thread and start your project. You need to remove the metal stopper at the bottom of the zipper....similar to a staple. Dispose of it as you will not be able to put it back on. Take the zipper all the way down, align the teeth then rezip to the top. Hold it carefully as you do not want to split the zipper back open. At the bottom near where you removed the original stopper so it closed in a smiliar fashion to hold the zipper teeth together so they will not split again.
Oh, how frustrating a separated zipper can be. Especially in a public place, oh me oh my not a fun time but have no fear an easy fix is here! You can find more information here: http://kerneltrap.org/node/17158