What is the ruling on a marriage contract that took place between a man and a woman in the presence of two witnesses and the woman's guardian, after they had been living together without marriage?

A man's living with a non-mahram woman is a reprehensible act due to the seclusion (khalwa) that may ensue, which could lead to immorality. Islam has forbidden even the mere seclusion between them.

As for the marriage that took place: if it was attended by two just witnesses, involved an indicative صيغة ( صيغة refers to the pronouncement or wording of the marriage contract, which explicitly states the offer and acceptance.), and a dowry, then it is valid according to Abu Hanifa, but invalid according to the majority of scholars due to the absence of a guardian (wali). It is obligatory to annul it. For it to be valid, the woman's guardian or his permission must be present. If there is no guardian and no legitimate Islamic judge, the woman is permitted to appoint a Muslim man to be her guardian, and the contract will then be valid.