Photo by Joerg Moellenkamp.
Test a small area of the fabric to see if it damaged by Windex. If not soak the fabric around the zipper with Windex and slowly, as in infinite patience, pull the zipper in the direction to unzip. Go a little past where you think it won’t go any more then relax your grip. Repeat — each time the zipper should move a little tiny bit further. If at any time the fabric bunches that you can see a tug on it might help things then give it a hard tug while maintaining tension on the zipper, this is not the time to be delicate, you are trying to pull the fabric out from the inside of the zipper mechanism. The Windex is a good, non-oil based lubricant.
I’ve done this more than a few times with success, from thick plastic zippers to metal ones. By soak I mean flood the fabric — the more the better. Don’t forget to wash the fabric immediately afterwards.