You will encounter this error if there a POWER issue with your ONU.
If there is No Lights on your ONU. This can be resolved by :
1. Changing the power socket or the plug it self (if you have an additional plug)
If the lights are on but the LOS light is RED and BLINKING,
1. Reboot the ONU (if the problem continues , go to step 2)
2. Turn the ONU around and check if the Fibre Connector is fixed firmly
Below is how a ONU looks like. (depending on which multiverse you are from)