Not sure if TT will share with you all the details of what’s transpired, but as noted a LOS event is pretty catastrophic - effectively no light whatsoever coming to you.

Typically this happens if there’s a complete break in the optical network between you and the handover exchange OLT. Either something affecting just your individual ‘drop’ from the CBT on the street or something affecting that CBT (4 to 12 customers) or indeed more serious affecting either the entire PON (up to 30 customer connections, on the same optic). You could I guess be the only customer on the PON (not terribly likely but not impossible) or it’s more likely just a problem affecting just your drop connection or your ONT has fault. A spare ONT would at least indicate if you’re getting light (even if it wouldn’t authenticate PON light blinking or off completely), but folks really aren’t likely to have spare Openreach ONTs in the cupboard.

I’ve not heard of this happening as a matter of a speed regrade or indeed an ISP migration. So it’s a bit of a mystery what’s transpired.

I'd at this point just power cycle the ont; it probably will do diddly squat but its worth just ruling that out .

I think he’s already done that, as said, “a handful of times”

My bad - slight case of forum blindness

Quick update - No update well actually - TalkTalk have claimed they have informed Openreach but no engineer details confirmed. Still in the dark, with the ONT and my router all glowing that eery red colour due to no service.

My closest neighbours are all served from the same CBT, and all have FTTP that still works so i do wonder if the ONT has got fried somehow during the profile change. The logs to the router clearly show it just drop off a cliff at 2am last week.

In reply to a post by Whitehall11:

My closest neighbours are all served from the same CBT, and all have FTTP that still works so i do wonder if the ONT has got fried somehow during the profile change. The logs to the router clearly show it just drop off a cliff at 2am last week.

personally and others will probably say otherwise, i'd probaby put fault in this order

1. ont
2. cbt plug issue - as in loose
3. csp
4. drop cable issue
5. splitter issue ....
6 olt issue .

but it does look like a hardware or physical issue..

Knowing my luck it won't be, anyways i hope you have a quick reconnect Whitehall11

UPDATE:
After being put round in circles by TalkTalk we are now back online, with PON light showing green and healthy and no red to be seen.

Now, the plot thickens here. This was all supposed to lead to a regrade of what i assumed to be 900/110 down, but the EERO router, as well as my talktalk hub (when plugged indepedently in) show 515 down and 110 up.

It appears we've got the higher upload but the old profile on the download.

TalkTalk support didn't quite understand this, and just said i need to wait for it to be activated?

What do you mean by the "router[s] ... show 515 down and 110 up"?

If you mean you've done a speedtest on an end device like a laptop (which is behind the router), then that device itself could well be the bottleneck - especially if it runs Windows.

Having said that, it's not impossible that Talktalk have applied the wrong BRAS profile and are shaping in the downstream direction only - but the router can't tell that, so you'll need to provide some more direct evidence.

Run a Thinkbroadband speedtest, preferably from multiple different devices connected by wired ethernet, and share the links.

In reply to a post by candlerb:

What do you mean by the "router[s] ... show 515 down and 110 up"?

If you mean you've done a speedtest on an end device like a laptop (which is behind the router), then that device itself could well be the bottleneck - especially if it runs Windows.

Having said that, it's not impossible that Talktalk have applied the wrong BRAS profile and are shaping in the downstream direction only - but the router can't tell that, so you'll need to provide some more direct evidence.

Run a Thinkbroadband speedtest, preferably from multiple different devices connected by wired ethernet, and share the links.

Was seeing the results from the EERO Router and TalkTalk routers connected directly into the ONT to check the speed. Annoyingly i don't have a ethernet connection for my devices as they're all newish - but plz see this screenshot of the EERO's built in test:
https://ibb.co/ZKnqGKx

The EERO is showing a 1 GBPS throughput

Good news you have green lights again on the ONT

The Eero box will connect to the ONT at 1Gbps link rate on Ethernet. This is correct and as expected.

If the eero is only reporting ~515 Mbps throughout on download, then that suggests you have been put on the wrong profile by TT.

Check again in the morning, as they may have applied a change overnight. Otherwise it’s time to get back onto support…

[also invest £10 in a USB-C/Thunderbolt GigE Ethernet adapter. Very useful to have and you will actually see the speeds at the top end of your 900 Mbps service (once working!) that you wouldn’t otherwise on WiFi]