Some GFX/Monitor specs would be handy here but hey ho...

This could be the GPU making incorrect decisions (simply put) which leads to it starting the output too low down, or it could be the CRT 'misfiring'. If LCD it should be a computer (GPU) problem. Try this:

Right click on your desktop, click properties.

Open the Settings tab and slide the slider to the very bottom, for the worst resolution (640x480 or 800x600 most likely) then click apply.

Now you can either restart the computer or skip to the next step. I'd restart.

Choose your original resolution and click apply. This should reset the GFX unit whether on-board or a seperate card and it should start applying the colour to the right part of the screen.

Of course do the daft stuff like unplugging the lead and putting it back in solidly.

If it persists try the monitor with another PC or the GFX card (unless you use on-board) with another monitor.