The DVD/CD can only be ejected if the screen is in DVD mode. You can check this using the Input button on the remote control or on the operating panel on the reverse side of the screen.
If the DVD is not ejected, disconnect the power from the screen for at least 5 minutes. You can do this by turning off the mains switch on the reverse side of the screen. Once the power is reconnected, the DVD/CD should be ejected automatically.