/Maybe you should post both the render and a screengrab of the preview to get a better idea. Bear in mind that the preview is exactly thid: a preview, and should not be considered final. It is very small and always darker than your final render will be. Do you alo mean tht it didn't happen before? Could it be that you use a post render effect? Double click your main camera and it will bring the post render options. Check if there is something there you may have ticked by accident.