there are many causes for decreased libido...they may be physical or psychological...physical causes may be due to hypogonadism resulting in low levels of sex hormones, not enough fore play,certain prescribed medications like antidepressants, high blood pressure medications, sedatives, antihistamines and certain birth control pills. ...for physical causes you need to consult physician or endocrinologist for detailed examination and evaluation with relevant investigations... If its psychological Consult nearest psychiatrist they will do the needful..