If the teeth are bent out of shape, there's likely no hope for it! But if it's stuck from age--maybe a little rusted or something , I'd try using a bit of oil (sewing machine oil if you have it) in an wee eyedropper, unless it has a very narrow spout (so you don't get any on the fabric) and inject a couple tiny drops into each side of the openings on the metal slide. Then take a pair of pliers to hold the zipper by its tab, hold the zipper & fabric taut as possible and gently pull the tab down. The pliers give a much better grip than using your fingers and you don't have to yank & pull. This has worked for me two or three times, both on dress zippers & purse zippers. Sometimes it hasn't, but it's worth a try, I think. Once it unzips, if it does, check the teeth, and you can sometimes shift any where the entire tooth is just out of alignment on the fabric of the zipper. But if the tooth itself is bent/damaged, then it's probably a goner.