How do you fix a headphones is only one is working?

The cable from the working speaker to the unworking one is cut, or the speaker is broken, how to fix it? Open the unworking one, check if there are two cables connect to two points on the speaker, if so then it should be a cut somewhere else between this speaker and the other side, find the cut and remove it, then reconnect the wires , make sure no wires can possibly touch other wires except there own colored wire, otherwise it will create a short circuit, try using tape for that.

Another option is to see whether it is a software error. To check this, right click the Sound icon on the lower left-hand side of the screen, or the taskbar (this only works in Windows 7). Select the option "Sound", and change to the "Playback" tab. Select your headphones from the list displayed, and click the "Properties" button that comes to life. Switch to the "Levels" tab and click the "Balance" button. If the balances are unequal, equalize them. If they are already equal, then, bad luck, you have a hardware problem and should try the fix above.