I hope I didn't miss the answer for reading too fast but I'd be willing to bet it was aluminum wire? It's unfortunate they use that a lot for the main run between the meter and the house and it only lasts 20 years or so before something like that happens.

They use aluminum wire to save money . . . . and that was back when copper was CHEAP!! YIKES! If they ever go back to it INSIDE the home, things will be even worse. It only half works for mains because the wire size is so large.