Try masturbating. See this for
guidance.
If it still persists, go see a doctor. If it has been more than 2-3 hours, call an ambulance and get ready to have blood being retracted from your penis. Possibly with a needle.
If it happens a lot and you still haven't seen a doctor, change your diet, check any medication you might be using.
If you're trolling, I'll personally come by and give you a 1 week mute. And when you think it is finally about to end, I'll give you another week. And I'll keep doing that until you actually cut it off or hit it with a hammer.