Premature greying of hair has many implicated factors like genetic factors, inadequate diet, stress, smoking, inadequate sleep or disturbed sleep cycle, etc. Try to take corrective measures in your lifestyle. Include adequate amount of protein, antioxidants in your diet.
Hair which have already turned white can not be reverted back, can only be camouflaged with hair dye. Treatment options available only target the hair in transition phase, I.e. they prevent further grey hairs from turning white. You can consult a dermatologist near you for better help regarding medications.