When a tight guitar string is stretched by a finger or pick (in preparation for plucking), it has been imparted with potential energy. When the string is released, the potential energy is converted to kinetic energy as the string is imparted with a vibrating motion. As time continues after the string is plucked, the kinetic energy of the string in converted into other types of energy, mostly sound and heat. The kinetic energy of the string is converted to sound waves, which is of course the entire purpose of plucking the string to begin with. The motion of the string also causes friction against surrounding air molecules, and this causes some of the energy to be lost as heat. The string would have to be plucked again in order to receive more potential energy.