How can I fix a broken zipper?

There are many different ways in which a zipper can be 'broken', all resulting in no longer being able to close it properly.

Do you notice any teeth missing?

Is the zipper made from metal, nylon, or plastic?

When you try to close the zipper, what exactly happens: does the zipper split open in the middle, does it get stuck part way to the end, etc?

If you could post a picture, that would be helpful in seeing what the cause of the problem might be.

Sometimes the fix on a zipper is simple. Other times it is not repairable, and would need to be replaced entirely. Although a shoe repair would be able to replace the zipper, it will be costly.

How old is the backpack? Is it a high end brand?

Flip turn is right, there are many reasons why a zipper may not close. The first thing I would do is see if there is anything stuck in it, that is the usual cause. If nothing is wrong and just stubborn. & nothing visible-look at it from the outside and inside of the backpack, a light shot of WD-40 at the point of it not closing may help. Before doing that, if you still have the store receipt, take it back and see if you can exchange it.

Here's one way to fix a broken zipper that I wrote about on my blog: https://mermaidsden.com/blog/fix-broken-zipper-bag

If it's broken in a different way, just let me know!

You can try rubbing pencil lead (graphite) on the teeth of the zipper. They sell graphite dust in a little bottle that you can also try like these - https://amzn.to/2XX7S7w. It works great in keyholes and sewing machines (what I have mine for) so I'm guessing it will work great on a zipper.