*Sometimes* I am able to fix these using a pair of needle-nosed pliers. Use the tips to pry open the side of the zipper pull that the teeth fell out of. Insert the line of teeth back into the side of the zipper and use the pliers to press it closed again. Then run the zipper up and down (you may need to use a little bit of force right at first) until both sides catch together.
Make sure when you do this that the zipper pull is ALL THE WAY at the bottom of the zipper so that both sides will line up.