Because beforehand the phone pair went from the exchange to the phone cabinet and from there to you.
With FTTC it goes from the exchange to the phone cabinet just the same, but is re-routed in there to the fibre cabinet, where the fibre signal is merged onto it. (The FTTC signal doesn't necessarily even come from your exchange). The result is routed back to the phone cabinet and in there is connected to the pair to your house.
What exactly happened I don't know, but maybe someone else's line was connected to your fibre and your line wasn't touched. The ADSLx would normally have continued working in that case so that probably isn't it. I'd need to think more, but that's how the circuit is constructed.
Edit - who was the line with before BT? That may give us a clue.
My broadband basic info/help site - www.robertos.me.uk.
Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 53.4 (interleaved)/15.6Mbps @ 600m. - IPv4BQM
Edited by RobertoS (Tue 24-Mar-15 16:05:10)