There comes a moment in every coat or jean zip lifespan where the zipper stops moving and gets stuck. This moment usually happens when you really don't want it to, like when you are getting ready for a big night out on the town. If you are at home, this is not as big of a deal, and a zipper can be unstuck pretty quickly. A tiny amount of Vaseline will be needed, though.
That Vaseline can be worked into the grooves of the zipper and the zip, and slowly but surely, it should ease up and start moving.