Hypothyroidism: The thyroid gland does not produce enough hormone.
Hypogonadism: Testicles do not produce enough testosterone.
Psychological causes: These may include depression, anxiety, or a panic disorder of some kind. (Difficulty achieving orgasm can also be a side effect of some antidepressant drugs, such as selective serotonin-reuptake inhibitors, or SSRIs.)
Physical injuries: Spinal cord injuries and other major wounds can have an effect.
Prostate problems: These include infections or surgery affecting the prostate or other pelvic organs.
These are some of the conditions that tests may turn up.
See your doctor or a sexual medicine specialist to find out the cause.