My buddy Carl had this exact thing happen on a trip to Monterey last year. His zoom lens got grit in it and wouldn't budge no matter what he tried. He took it apart with a tiny screwdriver set he bought online, but he snapped a ribbon cable inside. Ended up costing more to fix than the lens was worth, so maybe think twice before opening it up. He said the sand acts like grinding paste on those tiny gears, so forcing it just makes it worse.