There are many reasons for the loss of or absence of libido. Depression, stress, age, level of health, many drugs, lack of time for intimacy, all contribute to the loss of libido, including the availablity or not of a partner.
In addition, guilt, shame, and hormonal imbalances can contribute to the degree of a persons sexuality. Others are celibate for spiritual reasons.
If there is a desire to increase the libido, there seems to be many ways to facilitate that too. Interest has to be there...many times, in this day and age, the old adage, reach for your mate is replaced with reach for your plate or the remote....