Loss of cervical lordosis often called soldiers neck can happen from a couple ways....one holding ur head a certain way like soldiers do while standing at attention...another could be whip lash....even if it was yrs ago, can affect u as the muscles twist in the opposite direction can pull the cervical spine so it no longer has the curve...I had this and got my curve back...
It was a lot of PT on the muscles in the neck and shoulder area to retrain them back to the way they should be, this help reduce the pain I was having....
Look into Feldenkrais method and PT to work on the muscles as just having the bones back into place will not fix the way the muscles have changed...and will pop it right back to where it is ....it may be a slow process but it is possible to get some if not all of the curve back.
Loss of normal curvature can be a symptom of pain.
Do the ancient neck pain exercises the military has been using forever. You do what is an active stretch where you place your hand up to your forehead and push forward against the hand. The head is held in a normal straight up position, but the hand pushes against the forehead while the head pushes back. Hold for a count of 10. Repeat 10 times. Now place your hand on the side of your head and do the same. Then the back of your head. Finally the other side of your head. Do this daily and within days your pain will start to resolve. You are strengthening the muscles and balancing them.