Comcast actually shut off my Internet service mid-day when I wasn't home, because of this. I was lucky that I got an appointment the very next morning (I was on-call at the time) and the tech came out, found it was between my box and their equipment, climbed up a power pole, and immediately located a wire that had the insulation chewed through by a squirrel. They seemed very proactive about shutting my node off and then fixing the problem.

On the other hand, for all I know, the problem had been there for years.