When you right click on the desktop you get a display properties window, and on there is a settings tab.
Here you can change the screen resolution and color quality.
My guess is you have set these too high for your monitor so it has had to reduce the screen size.
Try dropping the screen resolution, so if it is 1280x1024 drop it down to the next resolution whatever that is, say 1024x768, and keep dripping it till it is now correct.
Or if the colour quality is set as Highest (32bit) drop it to Medium (16bit).
You wll have to play around with these settings till you get it back.
You do not say if it is a PC or laptop (my guess is laptop) but make sure you have installed the correct drivers for graphics card and monitor.
Laptops often need special drivers to handle the larger screen resolutions.