how old is the child?
Is the child sick?
Does this child usually have an appetite?
Is he/she lethargic?
Get to a doctor or hospital - dehydration can set in quickly.
You need to take your child to the doctor IMMEDIATELY. Not wanting to eat is not as much of a concern as not drinking is - a child can dehydrate very quickly and dehydration can be very serious, especially in a child. Dehydration will also happen a lot faster if the child is also vomiting and/or has diarrhea or a fever.
Please do not hesitate - take your child to the ER. Please let us know what you find out and how your child is doing. Best of luck!