A backpack zipper that won't stay closed is a slider problem in most cases. The slider — the part you pull — has two channels that squeeze the zipper coil together as it passes through; over time or with force, those channels spread apart and stop gripping.
Fixing a splayed slider
Use flat-nose pliers to gently squeeze the top and bottom of the slider body back toward each other, then test the zipper by running it closed. This works on a slider that's spread slightly but still intact. If the slider still won't grip after squeezing, it needs replacing — sold as a standalone part sized to the zipper's coil number (commonly #5, #8, or #10 on packs, marked on the original slider or teeth).
When the fix is bigger than the slider
Missing or bent coil teeth, or a zipper tape torn away from the fabric, aren't slider problems and won't respond to pliers. These need the zipper replaced by a gear repair service, since it involves unpicking and re-sewing the zipper tape into the panel — not something most home kits can do cleanly.
A dragging or sticking zipper
A zipper that still closes but drags or sticks partway usually just needs lubrication rather than a slider swap. Rub the coil lightly with candle wax, a dry bar of soap, or graphite from a pencil, then run the slider back and forth a few times to work it in. Avoid petroleum-based lubricants, which can attract more trail dirt and gum up the coil faster than they help.
Field workaround
If a zipper fails completely mid-trip, thread a cord, mini carabiner, or paracord pull through the remaining slider's pull hole to keep using it, or run a strip of tough tape like Gorilla Tape across the opening and pin the fabric closed on either side until you're off trail. Neither fix restores normal use of the pocket, but both keep contents secure long enough to finish the trip and make a proper repair at home.