When an upholstery zipper comes apart, what you want is an easy solution without zipper replacement. Provided the zipper teeth aren't damaged, the solution is relatively easy. Anyone familiar with operating a sewing machine can repair a salvageable upholstery zipper. There are two types of zippers: coil zippers (or self-repairing zippers) made of a spiral piece of nylon, and zippers with individual metal or plastic teeth. In general, non-separating upholstery zippers---the kind usually used in upholstered cushions---have few parts: zipper teeth set in cloth tape with a slider that draws the teeth open and closed.
Use needle nose pliers if repairing an off-track zipper slider on a zipper with a bottom stop. Remove the metal stop with the pliers. Discard the stop. Slide the zipper slider off the zipper teeth until it is free.
If the zipper has no bottom stop, rip any stitches that bind the end closed with a seam ripper. Slide the zipper slider off the zipper teeth until it is free.
Align the zipper teeth along the length of the cushion opening and pin the opening together.
Slide the slider onto the teeth until the teeth interlock. Ease the slider along the track until about ½ inch of closed zipper extends. Grasp the closed zipper end with the needle nose pliers in one hand and pull the zipper slider with the other. Continue closing the zipper until the cushion cover remains about two-thirds open, removing pins as the zipper slider slides along the track.
Hand stitch a bar tack over the closed zipper end to temporarily hold it in place.
Cut a 1 ½-inch length of twill tape. Lay the twill tape across the bar tack and pin it in place. Machine-stitch the twill tape to the cloth zipper tape as close to the zipper teeth as possible. Remove threads and turn the cushion right side out.
If the slider is lost, a replacement may be available. A number of zipper suppliers, speciality hardware stores and upholstery fabric suppliers sell zipper parts. Zipper sizes vary. Consult a zipper supplier or upholstery fabric supplier for parts.