The cartridge looks like a tongue and groove system, where the tongue or lip of the cartridge is installed first with the tongue in the groove for alignment. The cartridge then seats properly by pushing down on the rear top face of the cartridge into a slot in the back of the cartridge holder. It seems your cartridge has become stuck somewhere between sitting flush and fully inserted and in the installation stage.
Try pulling up on the back of the cartridge. If this doesn't work, try wiggling the cartridge side to side and gently pulling up. If this doesn't work, try wedging in a plastic giftcard around the cartridge and try to persuade the cartridge to move.
Good luck!