My ten year old son woke up with a stiff neck a few days ago. He couldn't move it at all!
He refused to go to the hospital saying that he slept in a bad position during the night. Sometimes he falls asleep at his desk and I have to wake him up to go to his bed. So it probably happened at that time.
I basically just gave him a heat pad to keep on his neck and a low dose pain reliever. It took a few hours for both to take effect but when they did, his neck loosened up very quickly. He woke perfectly fine the next day. I'm glad we didn't go to the hospital since it was nothing serious.