In very cold weather the latch can freeze in place. Attempting to pull the knob up by force will cause the magnetic catch to separate from the internal mechanism. At that point, there's nothing for it but to dismount the entire assembly, repair it and remount it.
All the pictures are on the following pages. Click the small images to see them larger. Click the left and right edges of the large images to step through a whole page as a slideshow. For printing or editing, fetch the high-resolution full-size image by clicking the Full-Size link at the bottom right of each large photo.