Having a child cry every night is a heartbreaker. I am so sorry that you are worried about her. Your daughter may be expressing her fears (for example, of being bullied, of failing school, of not being accepted by her peers). Another explanation is not wanting to leave you. Children will pick up on parents' feelings and act on those emotions.
Talk to the teacher and school counselor to find out how your daughter is behaving in school. What do they observe? What do they recommend?
Be supportive and encouraging. Listen to her concerns without judgment. You may even volunteer in the classroom and observe how she is doing.