If you look at the right side of the zipper, about two inches from the end, you will see that this zipper has lost several teeth. As a result of the loss of teeth, the zipper pull came off the right hand track. Therefore it can't be zipped.
This was a cheap pair of pants from Walmart -- brand "Lee Apparel." One doesn't necessarily expect superb quality from a discount store, but this pair of pants is less than six months old. I have to think that buying a more expensive pair would have paid off in longer life and therefore been worthwhile.
I should note that this is not the only zipper which has lost teeth in my home.
This didn't used to happen. Zippers did not lose teeth when I was a kid. Now we have planned obsolescence.