One of the most embarrassing misfortunes is to have one's zipper break open unexpectedly in public. Here is a very simple, inexpensive and fast step-by-step method to solve the problem on the spot.
Step 1: Slice the Zipper at the Bottom
After removing the garment to facilitate the process, use a pair of scissors or a sharp knife to carefully slice the zipper just behind the teeth. Do this first on the side that has detatched from the pull as near the bottom of the zipper as is practical. Be sure the cut is toward the top leaving a tail at the bottom of about the length of the zipper pull.
Step 2: Slide the Other Side Too
Now make a similiar cut on the other side if necessary. Be sure to make the cut directly across from the first so both tails are the same length and end together. The zipper will not work correctly if one tail hangs below the other. By this time you should be able to slide the pull down off the end of the zipper.
Step 3: Insert Both Tails Into the Pull
Insert each tail of the zipper into the pull as if zipping up a jacket. Start with one side inserted to the bottom of the pull. As you insert the other side, gently wiggle it in, up and down as you try to pull the zipper closed. With a little patience you should be able to find the spot where the two sides mesh. Once you have found it, zip it to the top.
The last step is to provide a means of preventing the pull from sliding back off the ends when undone. You could try wrapping a piece of sewing thread around the tails at the bottom and tying them off. I prefer to apply a bit of Super Glue as shown. Add only enough glue to make an effective zipper stop. You will still need to be able to open and close the zipper in practice.
Step 5: Ready to Go!
Well, that's it. You've now saved the day with an emergency zipper repair good enough to last until you can get to a seamstress to have it replaced. Congratulations!