Try recharging or replacing the battery pack.
Reset the units. Disconnect the battery pack of the parent unit and unplug both the units' electrical power, then plug them back in.
Is the unit turned on? Press and hold the ON/OFF button on the parent unit to turn it ON.
Is the correct camera selected? Change the camera number, if necessary.
Is the video display turned on? Press the VIDEO ON/OFF button to turn it on.
When operating using batteries only, the Display will go blank after 5 minutes or 30 minutes or 60 minutes to save power (depending on video on setting). The unit is still active and will give you audio alerts. You can reactivate the screen by pressing any button.