LOS means there is no connection of your fibre to the network (loss of signal) check that the fibre plug (green one) is fully seated in the ONT.
Also check that the fibre cable is undamaged, if it has been snagged then it can break inside.
Either way you will need to contact Sky to get it fixed,
https://www.sky.com/help/articles/contacting-sky
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I remember the Internet starting!
Sky Full Fibre & Virgin Media XGS-PON, with automatic failover using DrayTek multi WAN router.
Sky Q, which will be sadly missed when they decide to kill it off!
