Locate any obstruction in the zipper, such as a piece of debris, cloth or entangled hair.
Pull the zipper up and down gently and as you pull the stuck object out.
Open and close the zipper to make sure it's functional.
Locate the stop at the end of the zipper. (A stop that's broken or no longer functional can cause the slider to come off the teeth.)
Thread a needle. Sew a new stop from one side of the zipper's fabric to the other side. Sew several layers of thread to create a barrier where the slider can stop
Test the zipper to ensure it no longer comes off the teeth.
Pry the back of the slider open with a flathead screwdriver.
Move the slider over the zipper until it sits before the split area.
Crimp the bottom of the slider with pliers by applying even pressure on the slider.
Close and open the zipper by sliding it over the previously split area.
Loosen the slider with a flathead screwdriver if it's too tight, or crimp it tighter with pliers.