The Legend of Gratitude Tower

On his way to the examination in the capital, a poor student stayed at a girl's house for one night. Falling in love with the generous and clever girl, he promised to come back and marry her after the exam. As soon as he had passed the exam to become a new mandarin, he went back to the girl's house to keep his promise. But the girl was no longer there. What happened to her? What would the student do to return her love?