Left side of stereo earphone does not work.

In side of your left wire there are two tiny coper wires (either blue/green/Red and gold) one of which is easily cut off.
Peel off the wire cover (Dont cut it off) near the earphone head (normally not more than an inch) to findout where one of the colored wire is cut off and fix it.
Note: Before you could join or fix this broken one firstly you need to burn up the color coded wire with fire for a SECOND so that all the color (green/Red...) are cleaned up.

If this dont fix Please tell me. Im here to help you

Good Luck!

Hello, one thing to try if you haven't is to try the headset on another appliance to rule out the headset if you can, if not then the problem is either in the the phone jack or in the earphone itself, my experience has been that any time you try to replace the jack or open the earphone and try to reconnect the problem wire, it breaks, trying to replace the jack is useless since the wires are so fine it's next to impossible to make a right connection, as for the earphone you can try to open it up and reconnect the wire but since it may be short in the wire, it's a slim chance you will succeed but since it's not working you have little to lose, except time. It's your call, usually it's best to just replace it, it's not worth the frustration.