The same thing use to happen to me when I was younger. I would wake up with it, and could not move my head at all. Every time I moved my head it almost brought me to tears. I would miss 2-3 days of work becuase of it. The way I would get rid of it was:

- lay down somewhere comfortable because you are going to be there for a while.
- put a heating pad under your neck and make it as hot as you can stand without pain
- rub icy/hot on your neck every 30mins or so
- I took 2 aspirin and 2 ibuprofen every 4 hours (i'm not a doctor, this is just what I did)
- slowly move your head from side to side/up and down to work it out.

Usually took 3 days for me to get back to 100%. I'm getting a MRI next month for neck pain and headaches. Not sure if the neck lock and what I have now are related.