What if a young girl didn't want to get married or become a Nun in the Middle Ages?

In short, they’d be forced to work. A maid would be the most common job unless they’d managed to learn a craft skill such as cookery, sewing or basket-weaving. Even then, they’d be under the guidance of an employer. Women’s rights weren’t great so they would have been tied to a master or mistress which wouldn’t earn them much as they’d have to pay for their board as well. Their freedom would be reduced as they wouldn’t be able to have a dominating decision unless they had their own financial independence. Parents would try to steer them into marrying to create a better future for the family. Some would even abandon or disown them if they refused to work for their upkeep. Marriages could even be forced by arrangement. I f they didn’t agree they’d simply be shown the door to the outside world and wouldn’t be welcomed back and left to lead a life of beggary or prostitution.