Partial Power Outage No Breakers Flipped
There's only one thing in electrical that is octagon shaped: a stop sign.  No just kidding; an octagon box. 



This is a special type of box with a single purpose: mounting lamps in ceilings.  Your electrician clearly expects it to be a ceiling box, then; or, has seen evidence that the installer loved using octagon boxes for junction boxes.  

The box is trying to resemble round, but still be made by the same machines that make drawn steel boxes.  A 4" box is 4" on the diagonal (what it would be if it were round), somewhat less on flats.  This is confusing because "somewhat less" is actually 3.5" and 3.5" octagon boxes also exist.  Measure the diagonal.