With the locking pliers in the locked position: use the screw adjustment at the back to close on the zipper slider. (Make sure you're clamping just the zipper slider and NOT the zipper pulls or fabric.)

Remove the locking pliers and tighten the screw adjustment ever so slightly: a 1/16th or 1/8th of a turn should be enough: you're trying to apply the slightest pressure to both sides of the zipper slider when you re-apply it.

Return the locking pliers to the proper position on the zipper slider and clamp the locking mechanism down: this time, the locking pliers should lock just a little as you clamp down. Clamp the zipper slider on BOTH sides with the same setting before changing anything.

It might take a few tries with successively tighter settings on the locking pliers but eventually the zipper will start zipping properly again.