The Canon EF 35-70mm autofocus lens is designed to work for a number of years with minimal maintenance. However, at times the lens may stop working for one reason or another. If this happens, you can try a few things to get the lens working. If you decide to take the lens apart, contact Canon first to see if any warranty remains on the lens, or if Canon has issued a recall for the problem you are having.
Detach the lens from your camera and inspect the lens for any foreign objects or debris. Look for small debris between the sides of the lens. Sometimes sand or similar-sized debris can get wedged there and prevent the lens from operating.
Clean the metal contacts on the lens where it connects to the camera. Autofocus lenses receive electrical signals from the camera they are attached to. If these contacts are dirty, it may prevent the signals from being effectively transferred. Clean the contacts on the camera too, and replace or recharge the battery in the camera. To clean the contacts, use a cotton swab, moistened with rubbing alcohol. Remove any excess moisture with a microfiber lens-cleaning cloth.
Carefully lift off the mounting bracket. A ribbon cable will be attached to this bracket. Look inside the lens to locate the debris.
Use a pair of tweezers or a pencil with a piece of two-sided tape on the end to pick up the debris and remove it from the lens.
Replace the mounting bracket and screws.