In the best-case scenario, fixing a zipper that has come off its track involves simply pushing the teeth back inside the slider and zipping up. However, solving the problem may involve cutting the zipper tape.
The easiest method for repairing a zipper requires using either a narrow screwdriver or pliers. Move the slider down to the bottom of the zipper. Work the unattached teeth into the bottom of the slider. Use the screwdriver or pliers to work the teeth more fully into the slider. Zip the slider up to see if the teeth are now on track.
If the above method doesn't work, it may be necessary to cut the zipper tape. Move the slider all the way to the bottom. Using sharp scissors, make a cut between two teeth on the side of the zipper that's off-track. The cut should be just above the top of the slider and deep enough to accommodate the width of the slider. Work the bottom of the teeth into the slider, and zip up.
To repair the cut, select a thread color close to the zipper tape color. With the zipper closed, insert the threaded needle into the tape just above the slit. Work the needle under the teeth and up through the other side. Repeat this process until there's a thick wad of thread. This serves as the new stop for the zipper.