This is an abnormal, persistent, painful erection of the penis which, by definition, is unrelated to sexual desire. It is caused by certain blood disorders such as sickle cell anaemia which blocks the venous (blood flow outlet) system or by problems of the central nervous system that cause contraction of the corpora cavernosa, the blood spaces that fill and cause the hardening of the penis. If not dealt with impotence may result.
The problem must be dealt with by a urogenital specialist.
Complementary treatment is dependent upon the underlying cause.