spinal misalignment and/or strained tight muscles. Usually both.
The chiropractor can usually fix you right up. They really should manually relax the tight muscles, though, or else they will easily pull the bones back out of alignment. Sometimes you can find relief yourself by squeezing or pressing the tight muscles, forcing them to relax (hard to describe).
You can get out of whack by sleeping weird, turning your head suddenly, sneezing - pretty much anything.
If you go in to the chiropractor, ask them about your posture, make sure you have good posture in general, or what you could do to improve it.
Oh - pillow fights can cause outrageous kinks in the neck! And everywhere else! Beware! (Personal experience)