I have twice woken up with my neck frozen on one side. The first time it lasted for ca. two days and I was so frustarted I lay on the floor and tried to straighten it by hand, but failed. I then realised that I was stuck on the floor as the neck hurt too much for me to stand up. In the end I had to shuffle on my back to my desk and climb up a table leg. Eventually it wore off.
Second time was the day I was going on a scout night hike (some time ago all of this) - the frozen neck caused all sorts of comments. This time the problem was solved when I tripped over a fence in the dark and landed head first in some nettles.
So, my advice is the following: whne you have a stiff neck, walk into a low-lying fence, preferably close to some nettles.