Happens a lot in power conditions..... At a Guess your router is a Thompson Speedtouch or a rebranded one there normally dark blue in colour, Sky Broadband did use and rebranded these at some point....
It's certain firmware versions that does not save the username & password upon hard reboots of the router,
I see this day in day out in my Job always having customers ringing up saying we had a power cut and now internet is not working, in most cases it's the above fault......
Firmware update the router re enter all the config for the router then do a full config backup via it's own util, as it's easier to re upload a config file than having to go through the setup each time there's a power cut etc.
Also another known fault on some routers is that the built in firewall & intrusion prevention features get re enabled upon hard reboots of routers and this somtimes causes NONE NAT ADSL connections & in rare cases even NAT ADSL connections to fail as most ISPs have an auto detect feature that taps into your router and prediffines certain configs within your router.... The pure fact that yours didn't work last night after the power cut..... but this morning it does means with the router being on overnight at somepoint your ISP would have done somthing....
(im not saying that a guy at your ISP logged into your router to did somthing but more likely the ISP detected you were offline and ran a routine router check all ISP's have this sort of system in place but will never admit it)
somtimes you can check your logs on the router and you will find a random entry of a connection or in most cases a router reboot or DSL reset was done on your line.
Of Topic here now but info that's usful:
I'm with Virgin Media and often see activity within my router logs ok not DSL but still a external network.... so much so I enabled a syslogger server to log everything and found out that over a space of a month Virgin Media had accessd my router over 500 times on tcp & udp I looked into what they was doing and it turned out most of the connections were DNS address updates or what seemed to be, I whacked a firewall rule in place to block these connections from outside to inside after a few day's my service went down, I called tech support and after being passed to 3 tech people I finally got to someone that could answer my question.
They said they could not see my router from my cable modem to check things I said hang on one min turned off the firewal rule and told them to try again, they said they could now and what did i do I told them and they said that i was in breach of contract by blocking there connections and not to do it and that's why my service was stopped.
I was never given an reason to why they need access I can only persume that it's some sort of monitoring service they do to see what you are doing and accessing on the internet.
BIG BROTHER ALERT!
