I can hear audio from only one side of my headphones

Check the balance level on the source. If the source has balance level adjustment, it may be adjusted incorrectly. Make sure it is set to a middle level
Make sure the headphones are connected properly. The headphones may be connected improperly or the connection may have come loose. Remove and reseat the headphone connector
Try the additional audio cable that was included with your headphones (purchased after May 2012). If the issue is only specific to one cable, that cable should be replaced