Zipper failures are one of the top three repairs requested. Although they all start the same...my zipper isn't working...the failure could be caused by many factors:

Zipper is broken and must be replaced:

Symptom - Zipper teeth have fallen or been ripped out; or the zipper insertion piece is no longer functional or has been ripped out.

Zippers come in a number of sizes, #5, #8, and #10. Generally #5 zippers are used for jackets and not robust enough for packs; #8 zippers are most often used and are appropriate for all but the heaviest of pack uses...# 10 is the most robust and is often the best choice for extended trip and travel packs.