First, have the PoCo de-energize. While they're checking their connections, you can check yours at your service entrance, sub panels, etc. Just walk it logically backwards from where you're finding the low volts toward the source. Don't forget a wire can go bad in mid-span. Visually inspect aerials, and pray for your undergrounds. And, don't try to do any of this hot. Your "insulated" screwdrivers and wire strippers/crimpers are not safe to use on hot circuits.
Maybe you'll get lucky and PoCo will find a problem at their disconnect point and fix it in five minutes.