If you are using a VGA or DVI cable to connect your computer to your TV you probably need to adjust your screen resolution.

On Windows, this can be done by navigating to the control panel and going into display properties. Here you will see a list of screen resolutions and can test each one until you find one that works.

On a Mac you can go to System Preferences and select Display. Here you will see a list of screen resolutions you can try.

For both Windows and Mac ensure you select the correct screen if you are using "dual screen" mode. Normally if you are using HDMI the correct screen resolution will automatically be selected but if not try the steps above.