Hi, I'd like to help you with your question.
A child who cries, has nightmares and withdrawals usually has been through a trauma of some sort. They may have witnessed something or been traumatized themselves. There are many things than can affect a child in traumatizing ways. And depending on the child's sensitivities, one child can be more traumatized than another or have different coping mechanisms. Here are some possible traumas:
An illness in the family or a close friend
Separation from a significant person i.e. parent who leaves
divorce of parents
death of someone close to them
a change in living circumstances such as a significant move to somewhere different than the child is used to
If you have tried to talk to your child and have not had any significant progress, do not give up. Being there for your child is important. You can find ways to be with them and offer support, even without talking. Take the child to a favorite place, spend time together fishing, getting an ice cream or in a park. Places and activities that help your child have a chance to bring up what is bothering them helps. Continue to attempt to comfort your child through physical contact like hugs or other ways such as sitting with them and just being there.
Your child would benefit from seeing a therapist. An evaluation to determine what may be happening will give you and your child answers and a way to address what is happening. To find a therapist, talk to your child's doctor for a referral. Or you can search on line at http://therapists.psychologytoday.com/rms/.
You can also help by learning what you can about childhood trauma. That way, you have information to pull from when helping your child. Here are some resources to help you:
Sam Feels Better Now! An Interactive Story for Children by Jill Osborne and Kevin Collier
Lonely, Sad and Angry: How to Help Your Unhappy Child by Barbara D. Ingersoll PhD and Sam Goldstein PhD
When I Feel Sad (Way I Feel Books) by Cornelia Maude Spelman and Kathy Parkinson
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped,