Help! Materials in cycles render as transparent

The material is just a simple diffused shader… but when it renders it shows a diffuse shader but when the background pops in at the end it goes transparent. How can I fix this?

Because you say the background pops up at the end, I am assuming you are adding the background in with the compositor. It is most likely like that because of the settings of the mix node (or whichever node you are using to combine the render with the image.)