If you right-click on your desktop, select PROPERTIES, what resolution settings are shown and how many colours ? If it's 640 X 480 and 16 colours, then that's the problem. If everything was OK with your previous monitor, then I am puzzled as to why the video settings should have changed. If this is the case, then you will probably have to reinstall your video adapter drivers
First of all, though, see if you are able to change the colours and resolution settings (32 bit true colour and 1024 X 768 are optimum settings for a 14" monitor.