Is the desktop background all the way to one side? Or are you simply talking about the need to change your screen resolution? A graphics driver issue maybe? What type of monitor do you have? The more information you give us, the better informed our answer will be.
In the meantime..
To make the desktop background larger, smaller, or simply make it fit the screen, try using the monitor buttons to adjust the screen location.
If you need to adjust resolution which can affect the size of the background, then RIGHT click on a blank area of the desktop, choose "Properties", then "Settings", then change the "Screen Resolution" to something that fits your needs.
And if the resolution settings have simply disappeared, please try installing the most current graphics driver for you computer.
Hope this helps.
I do not remember what I was doing before the onset.