I have an old Canon AE1 35mm camera. Recently I was using it in the mountains in sub-zero conditions. At some stage the shutter wouldn't release back, so I had to close it with the film advance lever. Now I am off the mountains it works fine. I assumed that some working parts like the material around the curtain got frozen stiff? I changed the battery in the mountain, but same issue. My question/s is...can sub-zero temperatures have this kind of effect on old cameras like mine? Is there a way to prevent that in future? Will the affected shots be rendered useless? Thank you for your time and expertise. Roger Shepherd.