What happens if a persons unknowingly performs fardh salath, even though he is not clean (ejaculated in the night)? Suppose he does wudhu, performs fajr and also recites quran in the same condition. Only later does he realizes that there was an ejaculation in the night after seeing his clothes.
The short answer is as Laconic said: If you prayed your prayers without noticing that you have ejaculated you should re-do all of them, after performing ghusl (having a bath or shower) once you found out that you have been praying in this state. Because cleanliness (tahara) is a major condition for the validity of your prayers! So your prayers are void unless you perform ghusl after janabah (ejaculation in your case).
A more difficult case is if you have been imam in a prayer, should the people who prayed behind you in congregation also re-do their prayers?
The majority of scholars say no, only the imam needs to re-do his prayers based on this hadith.
See also this fatwa on what should the ma'moomeen do, and this fatwa answering the question whether this ever happened to our prophet (peace be upon him). And this fatwa in Arabic summarizing both cases.
And Allah knows best!
If the person acknowledges that he is unclean after the prayer, he should make Alghosl (الغسل) then re-pray the prayer again (قضاء )