Maybe you should try talk to your girl and find out if anything happened in school.
Simple things like 1) friends laughed at her 2) incidents with friends or 3) child being 'said' by teacher due to some things child has done.
These are my personal experiences with my girls...
Last year my 3 year old doesn't want to go to her play school and keep crying...
After I asked her, she said 'teacher scolded her' so I had called the teacher and have a better understanding of what happened.
It was an incident of some 'snatch toys' and teacher just say my girl and did not scold her..
After knowing what happened, I explained to my girl and told her that it's wrong to snatch etc...while she said it was the other child who snatched from her and teacher 'scolded her' instead (my girl)..
So I told her that next time if similar thing happen, let the child have it while she can go inform the teacher..
It should be something that has happened in your child's school.
Hope you can find out what's wrong and hope she goes back to school soon