Does a girl who prayed against herself not to marry, and then repented, bear sin for not marrying until now? And what should she do to get married?

There is no শরীয়াহ (Islamic legal) proof that a woman who dies without marrying will be among the Houris of Paradise. Rather, a righteous woman – whether married or not – will be better than the Houris in Paradise. Marriage is Islamically recommended, and abstaining from it contradicts the Shar'iah because it is among the practices of the Messengers and through it, half of one's religion is fulfilled. There is no blame on the questioner after she reconsidered her plea to Allah not to marry. She should ask Allah to grant her a righteous husband and should not hesitate to accept someone whose religiosity and character she is pleased with, avoiding exaggeration in conditions. It is permissible for a woman to offer herself in marriage to a righteous man, with certain regulations and manners.