This could be because your main Transmitter is out of range and is not in communication with the Display. You can try moving the Display to a position where it is closer to the meter or where it gets a better signal.
If you continue to have problems, consider changing the batteries in your Transmitter - you may also need to re-pair the Transmitter to your Display
You can also check the physical connections. Where the sensor fits to the bottom of your Transmitter and make sure the sensor head is fitted securely.
For an LED sensor, please ensure the Velcro is secure against the LED Pulse on your meter.
For a CT sensor, please ensure that the CT Clip is tightly secured around the live cable on your meter connections.