1)Firstly, check the balance setting on your audio device. Make sure the balance slider is set in the middle, to give a even output to the right and left sides.
2)Check the earphone jack and see if there is any wires that have come loose or have disconnected. If they have get a lower powered soldering iron and solder them back on. If you can't open your jack apart to do this, then order a brand new jack.
3)Check if the wires that connect to the earphones are in good condition. If they are damaged and you can see the insulation plastic has come, get some PVC electrical tape and tape it up. If the wire is badly damaged they a useless, unless you take them to a repair shop. I recommend anyways to buy yourself another pair!
4)Get a flat head screwdriver and open the earphone that does not work. Doing this will help you find out if the speaker wire has come loose or disconnected from the mini speaker. If they have, get a soldering iron and solder wire and melt them back on. This is a common cause that can cause the earphone not to work.