Use a lubricant on the zipper teeth such as petroleum jelly or a clear bar of soap to fix it. If you are at home and candle wax is available, it is useful to fix jammed zippers too.
You need to straighten the zipper and put it under tension. Then slowly move the small thingy a little bit upwards and try again without loosening the tension. That should do the trick
I find this a common problem on metal teethed zips, not so much on plastic ones. Pulling the two sides of the zipper tight at the open end and keeping as much as the zipper as you can in a straight line usually solves any sticking issues, you might need to wobble the zip a little from side to size to release it, but don't directly pull it hard as you risk damaging the teeth can causing more problems.
For the really difficult ones, I find a graphite, a crayon the same colour as the material or a candle a good lubricant to loosen them up.
In 90% of my cases, the reason is a layer of stuck fabric inside the zipper pull tab. Happens when the pull runs over a piece of thin fabric from the jacket or similar while moving along the zipper.
To resolve this quickly and without having to take a close look, with one hand bend away the jammed fabric from the pull tab's path, and with the other hand pull hard enough on the zipper's pull tab to move it.
It works best when pulling the pull tab in the closing direction, as its opening on that side is larger, making it easier to run over a bit more fabric. And sometimes, there can be stuck fabric on both the body-facing and outside-facing sides of the zipper.