In reply to mike123:
Do you mean the slider has fallen off the zip? It's the part you pull to mesh both sections of the coils/teeth together.
If it's a lost/damaged slider this is an easy fix at home. First you need to know what type of zip you have on the jacket. More often than not the zip will be of a YKK Vislon type, especially if it's a more expensive brand. These zips are the ones that have plastic molded teeth rather than a plastic coil on both sides.
Next is the trickier task of finding what size the slider/zip is (Not the length of the zip). If you still have the original slider there is sometimes a number on the back of the slider, these no's roughly equate to the width of the meshed zip. I find a No. 5 zip is the most commonly used for jackets (you would find V5 on the back of the slider, ie. a Vislon size 5 zip).
Assuming it's not a 2 way zip (one that has 2 sliders and can be unzipped from the bottom once zipped up) then just go on ebay and search for a No5 slider for a Vislon zip, there a fair few available. Likewise if it's a coil zip or a larger size just look for those.
Once you have the slider there are plenty of videos showing how to fit the new slider onto the old zip, I just snip the top of one side of the zip (it has to be the correct side), slip the slider on and carefully super glue the cut fabric.
This all assumes that the teeth/coils of the zip are not damaged, then it's your original route of full repair. The slider replacement is such a quick ,easy and cheap option it's worth trying first. Plus you get the warm glow of satisfaction from fixing it yourself and extending the life of your gear.