Many times a broken zipper can be fixed. Examine the problem area on the zipper to make your determination. Most regular clothing zippers are made with plastic coils. Heavy-duty zippers have teeth made of molded plastic or metal. The teeth or coil are always attached to a fabric strip called 'zipper tape'.
For metal zippers, and sometimes with plastic zippers, if the teeth are crooked, causing the zipper to pull apart at this location, sometimes you can forcibly straighten out these teeth, and your zipper could be back in business.
Missing zipper pulls:
If the zipper pull has come off, see if you can use a pliers and put it back on, or use something else (in a pinch) as a pseudo zipper pull, such as a paper clip or safety pin. If this offends your sensibilities, check with your talented alterations person to see if they happen to stock any zipper pulls.
Sometimes, zippers will just simply 'split' where teeth have separated, perhaps because of the garment being too tight or something traumatic happened to the zipper. These can sometimes be worked back together by a 'trained professional', but it isn't easy.
Frayed zipper tape:
These zippers need to be replaced, as there is no easy fix to this problem. If you, or your talented alterations person is not successful at fixing a zipper, then, you will need to have it replaced. This can be much more costly than getting it fixed, as it involves ripping out the zipper and replacing it. So, do try and fix a broken zipper first!