I'm sure you have asked your child many times what is causing the crying.  Here are a few ideas that might help you narrow down the problem.

Look for patterns.  Is your child crying before, during or after school?  Is it on a particular day?  What happens just before or just after he/she cries?  Did this happen last year?  Based on what last year's teacher said, are there any similarities or differences to this year?  This will require some help from the classroom teacher.  Does he/she cry outside of school such as when he/she has to go some place he/she doesn't want to go?

Is there a pay off.  When your child cries, what happens?  Is your child trying to get something from you or avoid something.  For example, I couldn't read well, so I would make myself get the hiccups so I didn't have to read out loud.

Talk with the teacher.  What observations can she make?  Is the crying disruptive to the class?  What is she doing to help your child?  What can you do to help the teacher?  If you volunteer, does the crying become worse?

Contact the school's guidance counselor.  This person has been trained and is experienced in dealing with problems such as this.  See if you can set up an appointment to meet with her and maybe even your child's teacher.

Seek private counseling.  This could become expensive, but some insurance companies do cover the cost.  Also, check with your town's family counseling centers.  They often have programs or groups that deal with young children and their problems.  Sometimes a child will be more willing to open up to a stranger.

Get a physical.  There might be something physically wrong that could trigger the crying.  The doctor can also run blood tests to see if there is an imbalance that could trigger the crying.  For example, if she cries when she becomes overly tired, a blood test might pick up why.

Talk to other parents.  Sometimes an objective eye can see things that we over look.  Ask them if they see your child crying and when?

Keep the lines of communication open.  Your child needs to know that you care and what to help solve the problem.  

Your child is lucky to have you on his/her side.  Keep track of your findings as they might hold the key to why.  And once you know that, you will be better prepared to help your child.