electrical anything

I receive many inquiries from people about their electricity not working.
I like to help, but the possible cause of problems can be drawn from so varied a list that it is nearly impossible to provide a useful response.
The following is a perfefct example.

Question:

I live in a 100 plus old six unit building in DC that--like many places--is staggering under the demands of modern devices. This has caused the occasional interruption with toaster oven plus space heater combo...nothing that can't be fixed easily and then thrown back on with the circuit breaker switch.

This week, however, it seems the whole thing expired. In one whole half of my apartment, no lights come on DESPITE the circuit breaker being clearly in the "on" position. In fact, it never flipped into the red "off" side. I've switched it back and forth a handful of times in the hopes that I can activate something, somehow, to no avail.

My unit has not been rewired completely ever, to my knowledge. a few outlets no longer work, but this is the first time half the building has gone out.

Each unit controls its own lights, room by room in the circuit panel, however there is a big box in the basement for the building, although when I checked it out, there was no labeling that indicated it controlled anything in individual units.

Answer:
There are near endless possible causes.
You need an electrician because:
1-Possible utility problem-partial loss of utility lateral or bad treansformer or connection failure
2-Partial loss of service entrance at metering or panel, disconnect or other
3-Blown fuse at service entrance panel or subfeed disconnect
4-Failed connection
5-Bad breaker
6-Local failure-opened circuit due to conductor termination failure
In short, anywhere there is a connection, a device or a current limiter of any type there may be a failure, or an open caused by a properly functioning safety device.
Note: The circuit open may not necessarily be in the ungrounded [hot] conductor