Whose loss? Regardless of who lost his/her sex drive, the evaluation is similar: first evaluate your relationship. Second, consider external stressors, eg money, family, friends, work, etc. Third, consider substance use, eg tobacco, alcohol & illicit materials. Fourth, any new medications? Fifth, evaluate his/her health: depression, diabetes, high blood pressure and/or cholesterol, heart disease, etc.
Age. How old are you do you have any medical condition that is affecting your desire are you abusing any drugs any history of trauma.