This is likely because Blender and Cities: Skylines don't agree on what is up. In Blender the z axis is up, in Cities it's y. 
Try opening the file in Blender again. In object mode you right click on the model to select it, then press ctrl + A and select rotation and scale. Then export it again as fbx and try loading it into CS again.
Try opening the file in Blender again. In object mode you right click on the model to select it, then press ctrl + A and select rotation and scale. Then export it again as fbx and try loading it into CS again.