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CAUTION - NOT for use on energized wiring. Doing so will result in a damaged tester and possible injury.

FIRST TURN OFF THE POWER. If you are not sure which circuit breaker or fuse it is, turn them all off or remove all the fuses. Resetting your clocks is safer and a lot less painful than a good jolt.
Every 3-way switch circuit will have two 3-way switches. If there are more than two switches in the circuit then the others are 4-way switches. There will be three wires connected to each 3-way switch and four wires connected to each 4-way switch. Depending on the age of your home there may also be a bare or green ground wire connected to a green screw on the switch.

You will not see all black wires connected to your switches as in the below examples and we have done this to illustrate that you can not go by the color of the wire or the physical location of the terminals on 3-way switches. There will be more than one cable also. Don't worry about the color of insulation on the wire right now, we will deal with that later. The switch wizard doesn't care, the switches don't care, and right now I don't think you do either, lets just solve your wiring problem.

Remove all the switches that are for the light that is not working. You don't need to remember which wire was on which terminal. The green or bare ground wire, if there is one, is not used in any of the wiring tests and should not be connected to any of the other wires. It is therefore not shown in the examples.

step one image

At one of the 3-way switch locations connect the three switch wires together, as in ( 3 way # 2 ) example above.
If there are any 4 way switches connect the four wires together at all the 4way switch locations as in ( 4 way all ) example above.
At the other 3-way switch location connect the 3 tester leads to the three switch wires, it does not matter which lead goes to which wire. Press the button on the tester and two LEDs will light. Mark the wire connected to the tester lead that does not have a light as in ( 3 way #1 ) example above. Use a piece of tape, bubble gum or whatever, I prefer to just put a loop in it.Connect the remaining two wires back together as in ( 3 way # 1 ) example below.

* If only the left LED lights then you do not have all the wires at the other switches connected together good.

second test

Go to the other 3-way switch location and connect the three tester leads to the three switch wires. Push the button on the tester and mark the wire connected to the tester lead that does not have a light.
* You must have the two unmarked wires at the other 3-way connected together.
If there are not any 4 way switches skip to step 5.

third test

If any 4 way switches are involved, Connect the remaining two 3-way switch wires back together as in ( 3 way # 2 ) example above. Go to a 4 way switch location. Connect the three tester leads to any three of the four switch wires as in ( 4 way all ) example above and press the tester button. Mark the two wires that do have a light.
* You must have the two unmarked wires at both of the 3-way locations connected together.
Now connect the 4-way switch, the two wires you marked go to the terminals towards one end of the switch, the screws will both be the same color. The remaining two wires go to the screws on the other end. connect the ground wire if there is one to the green screw. If there are more 4 way switch locations repeat this step at the remaining 4-way switch locations, marking two wires at each location and install the 4-way switch.

Now connect the 3-way switches. If there is a green or bare wire connect it to the green screw. Then connect the marked wire to the odd colored screw, usually black or copper colored and marked common. Then connect the other two wires, one to each of the remaining two screws, it does not matter which one to which screw.
If there are any white wires connected to the switch terminals the National Electrical Code suggests they should be re identified with black tape or paint to distinguish them from a white neutral wire.

Install the switches into the box being careful the ground wire does not loop up and touch any of the switch terminals. Put the cover plate on, turn on the power and you're done.

Your 3-way switch installation is finished and your hallway switches or stairway switches are now fixed, enjoy.