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 radiosignal.
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 over the pulse LED on your meter.