What you are looking for is the "Show Desktop" icon which should be in your Quick Launch Toolbar (QLT).
Two things to try:-
First of all, do you have two small right-facing arrows to the right of the other icons in your QLT? These indicate that there are other icons hidden to the right. Click exactly on these arrows and you'll see the hidden icons. If your little blue rectangle is there just drag 'n' drop it to where you want it to be.
Second, if you can't find it by the method above, go to Start -- Search -- For Files and Folders. Then click the down arrow at the side of Search Results and click on Computer. Then in the Search box type Show Desktop. When you see your blue rectangle, right click on it and go to Open File Location. You can then restore it from wherever it is to your Desktop or your QLT.