If the battery don't charge after a long period being used, you can :
Take out the battery from the headphones
Plus the headphone without battery with USB
When the LED light switch on, plug the battery, a "clic" can be heard when the battery is correctly inserted.
This procedure can be repeated 2 times before working. If the battery doesn't work despite several trials, unplug the headphone and reach your retailler to know more about your warranty and battery replacement possibilities.