I had the same issue after it had worked perfectly for about 9 months. Working one minute and then only showed my background on the tv screen the next.
Finally found the answer.
Click on the arrow in the bottom right of your screen that shows the hidden icons. Click on the one that is your graphics card. Then click on the option that says "Customize which programs use the GPU". Then click on the "Setup Multiple Displays" on the left. Under number 2 you need to right click on the Screen that is your TV and then choose "Duplicate Displays (Use 1 as Source)" then click apply and it should work.
Hope it helps.