Hilchos Lashon Hara chapter 4, section 12
If someone spoke Lashon Hara about someone else and now wants to do Teshuvah, doing that Teshuvah is conditional, if the people who were listening to him did not believe what he had said and the person who was the subject of the Lashon Hara was not denigrated or diminished at all in their esteem, then the sin becomes one that is (only) an issue between the speaker and G-d, the remedy is doing Teshuvah, meaning, remorse for having committed a sin before G-d, admission of one’s sin and a sincere resolve never to repeat it again.
But if the subject of his Lashon Hara was demeaned in the esteem of those who listened to the speaker’s comments and it resulted in damages to this subject or to his assets or caused him emotional discomfort and pain, then the sin is like any other sin man commits against another man. Even Yom Kippur and the speaker’s own death will not be enough to absolve him of his crime unless this speaker goes to the person who is the subject of the Lashon Hara and asks forgiveness. If the “victim” is appeased and forgives the speaker, then the only remaining sin is the one between him and G-d. Even if the person who is the subject of the Lashon Hara is still unaware of the speaker’s remarks, the speaker must go over to this person and tell him that what he did was halachically wrong and asks forgiveness. From here we can understand how careful a person must be to avoid this terrible trait because someone who is rooted in transgressing this esur, G-d forbid, will find it almost impossible for him to recall all of the people whom he harmed with his comments. Even if the speaker recalled all of their identities, because they knew nothing of his actions against them, he would be too embarrassed to approach them and disclose what he had done to them, and asks them forgiveness. So also, there are times when an entire family is degraded and thereby becomes injured for generations, and if that happens it will not be possible for this speaker to ever receive atonement, as Chazal have taught, one who degrades a family has no possibility of receiving atonement – ever!
Therefore one must stay very far away from this terrible trait because if not, G-d forbid, it will be a sin that will never have any remedy.