Have you changed monitors?
If its an older monitor, it may not be able to run 1024x768, or it may be not running at 100%, have you tried a different monitor?

Had the monitor you are using worked at 1024x768 setting before? I am trying to find what has changed since it last showed 1024x768.

Do you know how to check the settings and change it to 1024x768? see what setting its at now, try to change it, saving the change and restarting your computer