Originally Posted by nealtw
In one of those switch or plug boxes you will find a loose wire nut, most likely white wires.
True. You're going to find somewhere in that string there is one that has come loose. Turn the breaker off and start opening boxes up. You may want to even tighten all screws while you're in there to prevent this from happening again. When they get jammed into the boxes, things come loose.
Great job testing the circuit!