Sit them down and tell them that they can tell you anything. Stress that you will stand by them and believe what they tell you.
No matter what you're involved in at the moment, this is more important. Find some private space and talk about the question. Make sure the child knows that you believe her, that you will protect her, and that she was right to tell you about this abuse.
Stay calm, listen attentively, and address the question as well as you can. Write down what the child told you. If you suspect abuse, call your local Child Abuse Center and tell them what the child told you. Most abuse centers have experts who can come to your house for Play Therapy, where they find out more from your child in a gentle, positive manner that is not traumatic. Play Therapy can also be conducted at the Child Abuse or Women's Center, in a safe and non-threatening environment.
If you do discover that the child was abused, your response will be critical. Make sure they know that they are safe telling you about their experiences. Make sure they know you believe them. Make sure they know that the abuse is not their fault. Contain most of your horror, because your child will not be certain how bad this is. They may use your reaction to gauge how traumatized they should feel. So give them gentle sympathy and encouragement and reassurrance.
If the abuser is in your house, get the child to a safe place. If you can't get the child away from the perpetrator immediately, call 9-1-1. When you have established safety, call your nearest Child Abuse Crisis Center. If there isn't one nearby, call one of the national or world-wide toll-free phone numbers. Get their advice. Then call the police, if you haven't already.
Many crisis centers will send an expert to help you go through the process of abuse recovery with your child. Remember that with proper and immediate action, and appropriate therapy (for your child and for you), the long-term outlook for healing from child abuse is very good.