Praise be to Allah.
Firstly: it is not permissible for a man or woman to undergo any surgical or other procedure that will permanently prevent them from having children.
One of these means that is forbidden is vasectomy, which is a surgical procedure in which the vas deferens (the two channels through which sperm travels to the bladder and then to the penis) is cut.
It says in a statement of the Islamic Fiqh Council: It is haram to permanently remove the ability to produce offspring for a man or a woman, which is what is known as sterilization, so long as there is no necessary reason, according to Islamic standards, for doing that.
This has been explained previously in the answers to questions no. 111969 and 47196 .
Secondly: With regard to reversing the vasectomy, this is a matter concerning which one must refer to medical specialists. If the doctors determine that there is no harm that would result from that, then you must undergo this procedure and restore the vas deferens to its natural state.
But if that is not possible, or it will result in harm, then you do not have to do anything, because harm cannot be removed by further harm.
As you did that as a result of ignorance of the ruling, then Allah, may He be exalted, will pardon you and there is no sin on you. Allah, may He be exalted, says (interpretation of the meaning): {And there is no blame upon you for that in which you have erred but [only for] what your hearts intended} [al-Ahzaab 33:5].
And you did not deliberately intend to commit sin.
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