Why is there a volume imbalance in one headphone?

Why is there a volume imbalance in one headphone?
I’ve had my Backbeat Fit headphones for a while, no problems. Within the past few hours however, the left side is considerably softer than the right, so much so that when the right is at a normal volume the left makes almost no noise whatsoever. How do I fix this? Is it a problem with the buttons or the headset itself?

ANSWERS: