I would recommend seeking medical help, if you haven't already. There could be a number of things causing the pain. If you have already been to a pediatrician or other dr. then let us know what happened. It could be as minor as she just held her head wrong at night and it is stiff, or something as serious as Chiari Malformation if this is an ongoing problem.
Hope all works out. Let us hear from you.
I certainly go to your local hospital's emergency room immediately. I cannot believe you thought to take the time to write about this situation.
This does sound serious and medical attention should have been given quite some time ago.
I really don't know but if there was no injury you know of and your daughter don't know either. I would go see her pediatrician because unexpained neck pain would be serious to me if it were my child.
Hope I Help