Is the monitor a CRT or TFT type?
Is it all brand new or somewhat second hand?
If it is a CRT the tube may be fading away or perhaps accidentally reset from the best display result. You can usually play with the settings with the buttons on the front of the monitor.
If it is TFT it may be that you are not setting it to its "native resolution".
Also the "display driver" may have been corrupted or removed so all you have is the default one from Windows.
If you recognise any of these things and can tell us a bit more about your computer, then someone is bound to come up with your answer.