Here are the nuts and bolts about whiplash.
Take a look at the picture of the x-ray below. This is what a neck looks like before a whiplash accident. The curve is suppose to be there. It's called a lordosis. It distributes weight equally between the two joints in the back of the neck and the disc in the front. It also provides strength and motion at the same time.
When you are in a whiplash accident the head is whipped backwards, as in the picture below
Then the head is whipped forward. The head is to heavy for the spine and the muscles are too slow to react. Your neck is at the mercy of the force of the accident, as in the picture below.
The ligaments become damaged or sprained and the muscles get strained. The sprain / strain heals with time but the spine may never go back into the normal lordic curvature,as seen in the picture below.
This is why untreated or mistreated whiplash is so problematic. When you loose the lordosis the weight of your head is no longer distrubited. The weight is forced onto the disc in the front of the neck. The disc can prematurely degenerate due to compression forces. The spinal cord and nerves are stretched leaving very little tolerance for movement. Movements that would otherwise be pain free can cause neck and arm pain, tingling, and numbness. The head is moved forward ahead of the spine and is no longer balanced. This causes chronic muscular strain, tension headaches, and neck and back pain. The only way to evaluate this is with an x-ray form a Chiropractor.

The way this must to be fixed is with Chiropractic adjustments and exercises to restore the lordosis. Below is a picture of a man doing exercises to restore the lordosis in his neck. Depending on the severity of the injury it may take months or years to restore the lordosis. If you've been in a whiplash accident and are not doing these exercises then your lordosis may never be corrected and you can have life long pain.
CD exercise
Loss of lordosis