Whiplash... If is not involving nerve damamge, is usually just over stretched neck muscles. The best way to "fix" it, is to do stretches like Keres said above. make sure you are stretching til is hurts Therapy isnt supposed to be fun. Do lots of stretching, if it still is REAL painful in a week you will have to see a doctor.
Massage will do wonders, not just for the sex life.
Ice, the miracle of ice, you might think ice hurts, yet it will stop the swelling.
Use ice for a half hour at a time. put ice bag in towel.