In all seriousness, I dropped my YN-560 by accident. Although the flash still worked, the zoom ceased to function. I attempted these repairs after being encouraged by a commenter (or two) who also provided a helpful link to instructions for repairing the zoom on a Nikon SB-600. I'm not a handy person at all and I don't even know for sure how the repair below worked, but it worked. Here's what I did.
Link to photos: http://picasaweb.google.com/creadvty/YongnuoYN560Repair#
Step 0: try to discharge the electricity in the capacitor. I set the flash to full power, pressed the test button, then immediately opened the battery cover. Then I waited several hours.
Step 1: remove the rubber covers on the sides of the flash head. I used a small flathead screwdriver to pry the cover off, which was glued to the flash head.
Step 2: remove the metal clips near the hinge. There are two metal clips on each side of the flash head. I removed them by prying them off with a small flathead screwdriver.
Step 3: remove the screws on the bottom of the flash head.
Step 4: pull the flash head apart. Mine seemed to have some sort of rubber glue holding it together.
When you pull the flash head apart, you'll find a piece of plastic that seems to be for holding the wires together and maybe protecting them from external elements (?).
Step 5: remove the 2 screws holding the flash head body to the top of the flash head cover (circled in red in the picture).
Step 6: you'll see that the top of the flash head body has a screw drive. I did not find any damage on the screw drive on my flash, so I was at a loss as to what to do to fix it. Not having anything better to do with it, I rolled the screw drive with my fingers (warning: it's oily) until the flash moved to the most zoomed position (with the flash bulb furthest away from the edge of the flash head) so that I could have the benefit of the high GN at a cost of decreased coverage.
When I put the flash back together, I noticed that the sound was a bit different, and I tested the zoom. I was shocked that the zoom worked.