Happened once when I was pax on United 747-400 from Sydney to SFO, a few years ago. Woke up in total darkness, to find a few crew members and a couple of travelling firemen crawling all over the place with torches.
Unfortunately we were mid pacific at the time, so I would guess that there were no closely available diversions, eventually they found the fault, as described on this incident, and we continued to destination. Scary half hour contemplating what an interesting choice the captain must have if smoke source could not be found. Continue until unflyable (Swissair) ? or night ditching in the Pacific ? Captain sounded a lot cooler than I felt on his PA's though.
