I have an old Vivitar 200mm f3.5 Nikon non-AI mount prime that I got off eBay almost two years ago. I've used and loved it... and then, it stopped focusing. Or rather, the focus no longer changes. It's an old lens, obviously, but cosmetically, it's in great shape. No indication that it has ever been dropped or abused, and I have certainly never dropped or abused it. My laptop gets more abuse than my glass! Does anybody know what might have gone wrong? Is it just a case of some gear inside getting worn out? Can that be fixed? Is it WORTH fixing? Is there some really obvious thing I'm missing? I really can't afford to take it in for repairs or to replace it at the moment, so I'm really hoping it's something really obvious or easily adjusted. I can post photos of the lens, but like I said, from the outside it looks just fine; I don't think looking at a picture of it will help.