My Penis Does Not Stand When I Do Sex And

my penis does not stand when i do sex and i do not any feelings to sex. I do many masterbation in my life but never had sex till age of 28 now i am of 28 years and i had tried of sex but my penis does not stand. but i can do masturbation even now. i think doing exessive masturbartion cause this problem. Is it big problem ?Dont i get rid ever this dieases?Please answer me?I think my penis is dead?