Brief Answer:
Psychological causes are important

Detailed Answer:
Hello.
Welcome to HCM.
This is Dr. Swarnava Dattagupta answering your query.

See, the problem you are having is called as Erectile Dysfunction. It is a common problem and I come across many such patients in my hospital and chamber. There are many reasons behind it. Psychological causes are important. Also diabetes, malignancy and local diseases can cause erectile dysfunction.
As treatment, firstly I should tell you to control the underlying diseases if any. If there is no such disease, do the following-

You can take drugs like sildenafil, tadalafil, alprostadil etc. They are very effective and are used extensively. You can discuss with your doctor for prescription.
You can do processes like jelqing and try penis vacuum pump devices if the above is unsatisfactory.
Final treatment can be surgical like prosthesis etc but they are hardly needed. Drug therapy should be enough.

Hope I have answered your question.
Contact me for any further queries.
Regards.

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