I had something very similar happen years ago, when half my home's
circuits went dead. I called my city electric company, they came out,
checked my main panel, went out to the transformer across the street,
climbed up, and found that the "temporary" splices or clamps put in
when the house's electric service was upgraded many, many years
before, had never been upgraded to "permanent" ones. Eventually
corrosion worked its way in and cut off one side of my 240V service.
Apparently the final electrical inspection had never been done, which
would have caught that error. That's what I recall of their
explanation. They replaced the defective splices at no charge to me.