I just fixed one with the same issue. I need tweezers or small needle nose pliers to pull the hinge pin (as far as possible) to the still attached side. In my case the right side. I then lifted the magnifier and slipped the spring arms under. Keeping tension on the spring, I push the magnifier in the closed position and align the hinge holes. Once aligned, I use tweezers and a small flathead screwdriver. Tweezers to align the hinge pin and the screwdriver to push it into place. That should be it. If you are missing the spring, I cannot help with that.
Kirk