try slightly opening the gap on the back of the slider with some small pliers or your pocket knife. if you are dealing with a plastic or brittle metal zipper, trying to open that slider is likely going to break it.
if you are able to get the zipper back on track then even out the zipper tracks on either side and pull the zipper to the fully open position. if you loosened the slider, now is the time to gently tighten it back down before zipping closed again.
if your zipper slides up and down the track, but does not close the zipper there are a few possible fixes.
if you have 