You must unintentionally hide these icons when you use something in the desktop. So you could do the following steps to recover it .
At first , you should click the right button of the mouse on the desktop , and then choose "Arrange Icons By ", at last ,select "Show Desktop Icons".
After these steps, your icons will show again .
Not all the icons are gone though, just the icons that I have downloaded in the past week. The icons before that are still there