After a run I did sit ups with my feet elevated on the gate. After that night my lower back was never the same. How can I fix this?

Back Pain after W/O. I assume you mean that your back feels strange, has pain or is somehow not feeling up to your normal state of health. Depending on how severe your symptoms are...Or if you also have burning or numbness i believe you may need to see your trusted medical provider. The simple answer is resting the muscles to make sure they recover. Taking anti-inflammatories such as aleve (naproxen) or advil is wise.