I like using small amounts of 3-1 oil.
Just apply small amounts of the thin oil along the length of the zip, and try and manipulate the zip, so that it works loose.
It may take some time, but just work the zip up and down the serrated edge.
I used this on a very old purse, that had a zip fastener that over time had become rusty and just would not move.
After around three hours of trying to work the zip loose, it eventually became operational again.