What are the prayer for someone who was forced to had sex? Or what should someone do to ask Allaah forgiveness?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If the questioner means that she was 'forced' to have sex and did not have a choice, like a woman who is raped, then in this case she is not sinful as Allaah says (which means): {But if anyone compels them to prostitution, then after such compulsion, Allaah is Oft-Forgiving, Most-Merciful.}[24:33]. Allaah further says (which means): {…except him who is forced thereto and whose heart is at rest with Faith…}[16:106]. Moreover, the Prophet said: "Allaah has forgiven for my nation what they do by mistake, or due to forgetfulness and what they are forced to do." [Ibn Maajah]. To conclude, if a man or a woman is forced to commit fornication or adultery, he/she is not sinful. However, a woman who is raped has to find out whether there is any conception in the womb by having three continuous menses before her husband has sex with her if she is married, according to the Maaliki and Hanbali schools.

Allaah knows best.