bwah... this just happened to me on friday, except it was a wedding dress that i had made, and it was the day before the wedding! we ended up taking the zipper out, and i replaced it with hook and eye clasps.

after examining the zipper it appeared that there was a "tooth" (does that make sense?) that was pushed too far forward, inhibiting the zipper from operating properly. maybe check to see if this is what happened? if so, you could probably push it back into place with the others...

good luck!

You can remove the zipper and install another. you can install hook and eye, you can add snaps, you can even sew in Velcro. Tamp in grommets and lace up the opening.....
I guess its just a matter of what you would like to do with it. :D

Could you return it?

shazam, it worked and took about thirty minutes. What to do if a zipper is separated and the slide won't pull down:
1. make the bottom of the zipper accessible; it may be hidden behind lining, but is not that hard to get out.
2. Cut the little plastic stops off of the top of the zipper, and pull the slide off of the top. Best with tiny scissors so you don't damage more of the zipper than is necessary.
3. Squeeze the sides of the slide slightly to make it work better next time--use pliers, of course.
4. Cut off the bottom of the zipper--for hidden zippers with plastic coil, this is easier and quicker than removing the bottom tab. Make sure it's even, though.
5. Thread the two cut haves of the zipper through the slide--this takes some fiddling, and I had to recut the bottom because of frayed ends. It also takes some thinking to get the orientation right because everything is sorta upside-down at this point. I used pliers to pull the slide up through the lining to the outside of the dress, then pulled it up to a place I was happy having the bottom of the zipper stop.
6. make a new stop by sewing a heavy band of thread across the bottom of the zipper.
7. Make a new top stop on each side of the zipper using super glue to dam up the coils.
Voila, done.