If a person becomes junub as the result of a wet dream or intercourse, then he must do ghusl, because Allaah says (interpretation of the meaning):
“O you who believe! Approach not As‑Salaah (the prayer) when you are in a drunken state until you know (the meaning) of what you utter, nor when you are in a state of Janaaba (i.e. in a state of sexual impurity and have not yet taken a bath), except when travelling on the road (without enough water, or just passing through a mosque), till you wash your whole body”
If he cannot do ghusl because there is no water available, or because he is unable to use it, then he may do tayammum and prayer, because Allaah says (interpretation of the meaning):
“But if you are ill or on a journey, or any of you comes after answering the call of nature, or you have been in contact with women (i.e. sexual intercourse), and you find no water, then perform Tayammum with clean earth and rub therewith your faces and hands. Allaah does not want to place you in difficulty, but He wants to purify you, and to complete His Favour to you that you may be thankful”
Similarly, if a person has a wet dream and is afraid that he will become sick if he uses cold water, and he cannot find anything to heat it with, or the water is far away from him and he cannot reach it, and he cannot find anyone to bring it to him, then he may do tayammum and pray.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (1/151): If a person is sick and cannot move, and he cannot find anyone to bring water to him, then he is like one who has no water, because he has no way to get the water.
If he has someone who can bring the water to him before the time for prayer ends, then he is like one who has water.
If he is afraid that the time for prayer will end before the water can be brought to him, then Ibn Abi Moosa said: He can do tayammum, and he does not have to repeat the prayer. This is the view of al-Hasan. End quote.
As for wudoo’, it cannot take the place of ghusl.
What you should have done, if you could not find anyone to help you to do ghusl, was to wash whatever you could of your body, such as the head, face, hands and feet, then do tayammum, and pray with this tayammum.
For more information please see the answer to question no. 71202.
Based on this, you have to repeat that prayer.
Maniy is taahir (pure) according to the more correct of the two scholarly opinions, so you do not have to wash or change your clothes. See question no. 2458.
We ask Allaah to give you well being in your religious and worldly affairs.
And Allaah knows best.