Having a low sex drive can be due to multiple reasons:
Sexual problems (ranging from erectile dysfunction and ejaculation problems to vaginal dryness and inability to orgasm)
Constant feelings of sadness or hopelessness. A complete lack of interest or desire in any activity you previously enjoyed taking part in.
If you always feel tired, or stressed out, it can have a major impact on your overall happiness (this includes your sex life). Having a chat with your doctor may be advisable.
Many people experience a lack of sex drive as they age. This can be due to lowe levels of sex hormones in men (testosterone) and women (oestrogen and testosterone) or specific age related conditions.
Longer term conditions may have an impact on your libido too. For instance, heart disease, diabetes or cancer.
Drinking excess alcohol over an extended period or the misuse of drugs can affect your sex drive too.
Low libido is usually indicated by having a decreased desire in sex, as there's no real scientific measurement for a lack of libido.
The libido resides in 2 parts of your brain, the cerebral cortex; the outer layer of the brain responsible for higher level planning and thinking (which also includes sex) and the limbic system; the hypothalamus, hippocampus, and other parts of the brain, which are involved with emotion, motivation, and sex drive. The cerebral cortex and limbic system are so crucial, that a man or woman can orgasm just by thinking (or dreaming) about a sexual experience.