Possible open neutral?

I have a series of lights that stopped working. Garage, laundry, bedrooms. Checked all breakers and they all test good and still operate something. No loose neutral in breaker box. Tried connecting a neutral directly from breaker to different switches but no help. I connected hot wire only, no neutral, to a couple of different switches and everything works. I tried to trace the wires in the ceiling but they all disappear into the walls. Holes were drilled into the 2x4's and foam sprayed around the wires. It's a mess. Same was done to the wires coming out of the breaker box. Can't move a wire to see which breaker it's connected to. Labels inside breaker box are a joke. Any ideas? I would run a new wire but don't want to burn the house down.