Having a broken zipper doesn't necessarily mean that you have to get rid of the garment or that you even need to take it to a tailor to be repaired. The zipper stop, which is the part of the zipper that moves, is designed to "knit" the zipper teeth together. If it comes off, the zipper can still be repaired as long as the teeth aren't bent and the zipper stop is not bent or broken.
Examine the zipper stop closely to make sure it is not be warped or broken. Slightly widen the opening on both sides of the zipper stop with a pair of needle-nose pliers.
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Match the sides of the zipper. Make sure that the ends of the zipper match exactly. Otherwise, when you put the zipper stop back on, it will not move along the zipper smoothly.
Put the zipper stop on one side of the zipper. Move it up and down the teeth to make sure it is on securely. Put the zipper on the second side of the zipper. Move it up both sets of teeth to make sure that it closes the teeth properly.
Squeeze the sides of the zipper stop to get them back to their original position. Try the zipper again to make sure you have not squeezed the zipper stop too much.
You can find replacement zipper stops at sewing supply stores or online retailers.