That lever on the top should move, likely forward. if it does and the mount doesn't come back up automatically, you may have seized springs, whatever you do DON'T get your fingers near the pinch points if that lever is released... Have a friend who lost the tip of his finger that way.
The alum sometimes corrodes together... not sure what'd break it up, but I'd probably spray some WD-40 on it if that is what is seized.
If the lever itself doesn't slide, a tap with the hammer might help free it. Again keep hands away from pinch points, the mount might spring up quickly without weight on it.