You can get a stiff neck from sleeping in a bad position or from a cold draft hitting your neck. If you have a stiff neck and a high fever, you could have a serious disease so you should see a doctor right away.
For a stiff neck without fever, soak a towel with natural apple cider vinegar and warm it in the microwave. Be sure that the towel is not burning hot. Wrap the towel around the sore area, and leave it on for an hour. Repeat if the pain is not gone. You could also use strips of cloth or large gauze bandages instead of a towel.
With a neck that is "out of whack" (or out of alignment), no amount of aspirin, advil, or tylenol will help the pain. Even the strongest pain medicine may barely take away this kind of pain. For this type of neck pain, take 800 to 1000 mg of valerian root. This is a natural relaxant, not a pain pill. During the day, you may have to take a second dose of valerian after a few hours. If you take the valerian at bedtime, you should wake up good as new and with no "drug hangover."
Do not mix valerian root with sedatives, antihistamines, or alcohol.