Penis erection has suddenly downed. no tension. what could it be?

Many possibilities: There are many causes of erectile dysfunction or impotence. Stress is very common cause, eg relationship w/partner, money, job, family, etc. Poor lifestyle choices like tobacco & excess alcohol also lead to ed. Chronic health disease & medications to treat such, eg high blood pressure, high cholesterol, diabetes, depression, hair loss, etc can cause ed. Go see your family doc for complete evaluati.