There are a variety of causes for a stiff neck but one common one I have reviewed over my career is referred to as a Wry neck. In my professional experience clients have awoken with an extremely stiff neck where they can not turn their head and are especially stiff to one direction. There can be a variety of different causes but the injury itself is not entirely well understood. However, even though it is not entirely understood, the joints and muscles do respond well to physiotherapy treatment. Please contact a physiotherapist if you think a Physio can assist you with your stiff neck.