What is the ruling on marriage if, during the marriage contract, it is stated that the wife is a virgin, but she is not? And is this marriage considered void?

Your fiancée is a stranger to you (i.e., not a mahram), and what occurred between you is zina (fornication/adultery), for which repentance is obligatory. It is not permissible for you to marry him until you know that he has repented. Furthermore, according to some scholars, your marriage is not valid until you have performed istibra' (a waiting period to ensure no pregnancy). The fact that the marriage contract was performed with you being a virgin does not affect its validity.