Have you tried the headphone is some other device to make sure it's not the headphone or a dirty plug on the headphone? Also wiggle the plug in the jack and see if you get the right side to work.
Finally, check the balance, if you have one in case it's set to left only. On my system, if I doubleclick on the speaker icon in the Sound Panel, another panel comes up and in the Levels section there is an option for L/R Balance.