When the internet dies, your modem is already telling you which layer broke — you just can't read it yet. LOS red means the light itself is gone: no amount of rebooting brings it back.

Why a light can tell you more than a reboot

Your connection is a four-stage chain: the provider's fiber all the way to your building (access), the fiber from the wall into your modem (drop), the cable from modem to router, and the Wi-Fi from router to device. A break anywhere kills the internet — but each stage leaves a different light behind. That's the whole trick: the lights are a status report written in colour, and most people never learn to read it.

Two of the three modem lights matter here. PON is green and steady when the modem has successfully registered with the provider's equipment — that's the “I'm in the network” light. LOS (loss of signal) is off when everything is fine and turns red when the fiber is delivering no light at all. So the counter-intuitive part: a dark LOS light is good news; a red one is the alarm. People see a dark light, assume the box is dead, and start rebooting something that was never broken.

Match the evidence

• LOS red, PON dark → the optical signal is gone: check the yellow fiber for a sharp bend or a crushed section (anything past 90° kills the light), wipe the connector end one way only with lens tissue, and reseat it until it clicks. Still red after two minutes? That's provider territory — call them. Rebooting will never fix this one.
• Modem lights fine, whole house still offline → the classic bad restart. Unplug both boxes, wait at least 30 seconds (a minute is better — the capacitors need to drain), power the modem first and wait for PON to go steady green, then the router. Doing it the other way round means the router tries to dial into a modem that isn't registered yet.
• Only one device offline → nothing upstream is broken; the modem and router are fine. Test a second device to prove it, then look at that one device: connected to the wrong Wi-Fi network, airplane mode, or a stale lease — forget the network and reconnect.

The line you should not cross: never open the splitter box in the hallway, never touch outdoor cabling, never re-terminate fiber yourself. The fiber core is thinner than a hair and a bad splice costs more than the visit. What you can do is exactly three things — read the lights, reseat a connector gently, and swap one network cable.