I've woken up and can't turn my head one way. What would cause this?

Muscle strain. Sleeping in abnormal cervical position. Motrin otc hot towels to neck. If no improvement by morning call physician. Muscle relaxant and stronger antiinflammatories are helpful.
Muscle tightness. If you sleep in same position for too long, muscles become tight like a cast on elbow would leave you in pain when cast removed and starting to move for first time.
Torticollis. Torticollis is a fancy, medical term for a stiff neck. This can be caused by a number of things including sore, stiff muscles or even mis-alignment of neck vertebrae. As a musculoskelatol problem, several different folks might be able to help including massage therapists and osteopathic physicians. If you have a stiff neck with a fever, you should see your doc to rule out a nasty infection.