The container frame is racked so that the door gear will not operate correctly. This may be caused by cargo shifting during transit. Look at the container to make sure that the doors are aligned and level, both top and bottom.
The hinge pins and blade are seized due to corrosion.
The door gasket has been damaged and is preventing opening. Door gaskets are designed to present two or more fins against the structure or adjacent door. These are generally flexible but when the gasket is damaged, they may become hard or blocked thus jamming the door closed, or preventing it being closed.
Water has become trapped between the doors and frozen, particularly relevant to refrigerated cargoes, or containers with moisture releasing cargoes in cold weather.