The sun lamp's position, inside the room, doesn't affect where the sun's light is actually emitted from, it only controls the direction. To quote the blender manual:
Sun lamps emit light in a given direction. Their position is not taken
into account; they are always located outside of the scene, infinitely
far away, and will not result in any distance falloff.
So the light is being cast onto the outside of your room and not getting through the walls to the inside. You can see this by looking at the outside of the room while in 'Rendered' mode of the 3D viewport.
To solve this, you can either disable the 'ceiling' from casting a shadow by changing the 'Ray Visibility' options on the 'Object' tab in the properties space:
Or, change the type of light from sun to something else, like 'Point' or 'Area', whose positions are taken into account. You can also just set any object to emit light, like a plane, by giving it an emission shader.