Have you pressed the key that controls screen mirroring? On Toshiba notebooks it is usually Function F5, or consult your manual.
On Windows 7, hit Windows-P or right-click on the desktop and choose Graphic Properties.
Or go into Control Panel and set up the display properties Appearance: Display: Connect to an external display.
Make sure that the source input on the external display is switched to the input that the computer is cabled into.