Had this problem after having to reinstall after a system crash.
At first I just thought it was the TV as I was having problems with the color on it before.
With the display attached:
- Right click on the desktop
- Select properties
- Click settings
- Click on the 2 in the display window to select the secondary display
- verify the color and size settings
- if these are ok click advanced
- click the monitor tab and verify the display type is correct