I was helping someone add a light fixture to a circuit. I opened a junction box in the basement to tap off. Inside this box were 3 romex (old). One black comes from panel, connect to the black of the 2nd romex, which continues off down circuit. The third romex had the white wire connected to the other 2 black (hot) - this goes to the light switch. I understand this. After trying to hook up a light into this arrangement, it would not work. Disconnected the new light from this wiring. NOW, this circuit does not work, but breaker is not tripping.

I realized later that I need to run a 14-3 wire from first light (where hot from panel comes in), to the 2nd light, then run wire from that light to switch. (I found a circuit diagram online that shows exactly my situation.

Why is this circuit not working. When I touched and AC voltage tester to the black wires and ground/metal box, it shows voltage. When I touched the tester to the white wires and ground/metal box, I get a voltage reading. It seems that everything in the box is hot. Again, I removed my "add-on" wire for the new light AND I removed the switch wire, to see if this made a difference. It does not.

My friend says their digital thermostat and outlets in an upstairs bedroom do not work now. HELP!

Thanks

That white connected to the blacks is hot going to the switch, and the switched leg that comes back from that switch will be the black of that same cable. I would bet a wire broke inside the box where it may have been nicked, and eventually broke?

I opened a junction box in the basement

Is that a true junction box or is it a ceiling box to which a light fixture (controlled by a wall switch) is attached. Sounds like a ceiling box for a fixture.

The black from the panel, the black going on down the line and the white to the switch should be connected together. The white from the panel , the white going down the line and the white from the fixture should be connected together. The black from the switch and the black from the fixture should be connected.

If you want to add another fixture and the fixture is in the same direction as the cable to the switch, you can change the switch cable between the two fixtures to 3-wire cable if both fixtures are to be controlled by the original switch. If the fixtures are to be independently controlled, just run 2-wire between fixtures and then wire an additional switch.

Sounds like you got your wires mixed up. That is to say you are mistaken about which wires come from the panel, which goes down the line and which goes to the switch. Either that or you simply have a bad wiring connection.