As stated, if you haven't had it too long you should complain to the seller. But it perhaps can be repaired depending on the problem.
If there are teeth missing, try to bring the two edges together and using the zipper itself thread it on and carefully bring it past the fault.
When this is done, glue the zipped part (where the teeth are missing) and let dry.
You should be able to use the mended zipper as usual but only from the broken teeth (glued) onwards.
Heather, May 2009