Inspect your product to understand the way the zipper has been constructed, and what the defect is.

In this example, the zipper sliding component is defective, and cannot slide properly anymore. It needs to be replaced.

Note: at this stage, it is important to determine which part of the zipper is broken. If there is something wrong with the zipper teeth (i.e., one tooth has snapped off) then this guide will not help solve the problem