Wallpaper Being Covered By Background Colour?

when i switched it on, i couldn't see my wallpaper, but just a black screen. I tried changing it back to the wallpaper, but even though the wallpaper seemed to change, i still could only see a black screen. The wallpaper is there, because when i close a game it appears for a second, then gets back to the black screen. It's like it is being covered by the background colour. Everything is still working fine, and i can use all the icons on my desktop, however a black desktop is very annoying