Crawling around under a desk to retrieve a fallen cable is never a fun task. There are hundreds if not thousands of product specially design to solve this problem. Some work (I’ve bought those) and some don’t work (I’ve bought those also). It wasn't until reading an article from one of my favorite websites (lifehacker.com) that I solved my cable problem with binder clips. Clamp the binder clips on the edge of your desk, monitor stand, or book shelf. Remove the handles, slide the cables into the handle, and then reattached the handles. Problem solved!
Above is a picture of my desk. The binder clip holds my network and power cable for my laptop.