She might have been bullied or teased, or it may have something to do with the separation anxiety, where she does not want to be away from you. My daughter has gone through both of those things, and still has a bit of the separation anxiety, meaning that she does not like me to do morning drop off at the curb, and insists that I walk her into the classroom. If this is something that may calm your daughter and make her happy, you may want to suggest doing that. It only takes a few minutes of your time.
I also try to use that time to talk to the teacher about issues/questions. I would suggest speaking to the teacher as well, maybe the teacher has observed something going on (like a kid who teased her), and she thought it was not important enough to email you about it, but if your child is having a phobia and you explain that to her, it may get the teacher to mention that incident. Is there a school counselor that may have observed her/spoken to her and may also be able to give you some information? Could she be nervous about school testing? I know FSA testing has a lot of kids nervous and anxious. If nothing has changed at home, I would definitely try looking at other things that are a part of her life (school, extracurricular activities), that may be causing her distress.
I am sure the psychiatrist will be able to get some information out of her, so see what he says, but if he thinks it is school-related, then I would email the teacher and speak to the counselor to see if they can fill you and the psychiatrist in on what is going on. By the way, when my child's teacher resigned and a new teacher stepped in, she was anxious about that, as were other kids. It could even be something as minor as a change like that, that is worrying her and making her refuse to want to go back to school. Good luck, and please fill us in if you can.