Whereupon you will present yourself to an Emergency Room where the lowest doctor on the totem (not scrotum) pole will attempt to relieve your misery. This may involve injecting something into your penis, draining the blood from your penis, a vigorous shaft squeeze, or combinations of the above, alone or simultaneously.
Article in the “weird news” many years ago about the fellow who (TMI) forced cocaine into the opening of his penis and then after a few days had to have it amputated. Too much blood not going anywhere for too long causes a serious problem.
One of the things viagra helps (er… or so I’ve heard ) is the situation where a man can get an erection with stimulation, but as soon as the stimulation stops, he loses the erection. So it gets to be a race when changing from, well, one form of stimulation to another.