Our nephew cried the entire day at school for over four months. He stopped crying after that period of time. Our nephew suffers from anxiety and the crying was symptomatic of separation anxiety. I suspect your daughter is crying because she is "terrified" - unrealistic terror is the main reason for anxiety.
I might suggest you google the phrase "separation anxiety and children" or "separation anxiety and school" or "behaviors of anxiety in children" to better understand this issue. It is possible that your daughter should not be attending school so young - I believe anxiety is a sign of emotional immaturity. I also believe anxiety is an inherited genetic trait and not caused by life experiences. Nonetheless, too much talking and couselling won't work for such a young child; what needs to happen is to lessen her fears/anxieties. Is there a special friend who could help her at school? Could a parent drop in for recess or lunch? Does your daughter take a "security" item to school as a picture of Mom or a special stuffed toy? Is the teacher educated on anxiety? I'm sure the reading I suggested on the internet as well as lists of books in bookstores and your public library should be able to provide you with additional ideas on how to decrease the terror your daughter finds in attending school alone. All the best ....
Check that she is happy at school that she is getting on okay with other children and the teachers,.What reason does she give when you ask her why she is upset ?