That looks like a very nice scene. Although there can be differences between the preview and final render due to detail levels, in your image it looks like the lighting is actually totally different. It's difficult to say what might be happening without having a project file to look at, but my guess would be you may have turned off some important setting in the renderer, for example raytraced shadows. You can try switching renderers or create a new renderer and hook it up to your camera - it will have default settings and you can then be sure it's nothing you have changed.
- Oshyan