A stuck or stiff zipper is usually a lubrication problem or debris caught in the teeth. Both fixes take under two minutes.
Look closely at the zipper teeth where the slider is stuck. Use tweezers to carefully pull out any threads, fabric, or debris caught in the teeth.
Run the tip of a graphite pencil along both sides of the zipper teeth above and below where the slider is stuck. Gently work the slider back and forth with small movements while applying slight tension to both sides of the zipper.
If graphite alone doesn't work, rub a small amount of lip balm or candle wax directly on the teeth. Work the slider again after applying.