I have this problem too. But I'm able to fix it by just plugging and unplugging the cord from the device that the headphones are plugged into.
'''Tip'''
Keep the headphones on while you are plugging and unplugging. Once you hear a pop-like sound, it means that the sound has been restored in BOTH sides of the headphones.
I don't think it matters if you have music or any sound playing while you're fiddling with the wire, but it is easier to hear the pop sound if there isn't anything playing.
If NO sound is playing even after you plugged it in and turned on the music, just charge the headphones.