Dr.I had this problem about more than 30 years .I...

Treatment of sexual problem:

It depends on the cause, so treatment may include conservative management as doing regular exercise, drinking good amount of fluid daily, good nutrition, avoidind stress, and you have to control diseases as dibetes, hypertension.
If there is no response you can take sildenafil or cialis.

Diagnosis of the cause of sexual problem:

You have to visit your doctor for clinical examination and investigation as CBC  to detect anemia and infection, glucose level, blood pressure measurement, renal and liver function tests, ECG and ECHO for cardiac problems, PSA for prostate problems, urine analysis...etc

Causes of sexual problem:

• Stress and anxiety
• Dibetes
• Hypertension
• Taking medications
• Prostate problems
• Hormonal problems as thyroid and testosterone problems...etc

What i advise you?

You have to visit your doctor for the right diagnosis and treatment.