Sometimes the contacts on your camera and/or lens get dirty...you may not see anything, but it can cause connectivity problems so that your camera may not recognize your lens. An easy way to clear this up is to rub the contacts with an eraser. You can also do this on battery contacts, charger contacts, or anywhere else that you see those little metal spots. If you are rubbing contacts inside the lens mount of the camera, be very careful not to get any eraser crumbs inside the camera, they could get under the mirror and get stuck on your sensor which will cause spots on your images. You can also rub the contacts with a Q-tip that has some Windex on it.
To clean your lens glass, a lens cloth works fine for minor marks like a finger smudge. For more intense jobs, first blow the glass element off (with canned air or a bulb blower) so that there is no harsh particles like sand or dust on it- it could scratch the glass during the next step if it's not first removed. Then, take a Q-tip with some Windex on the tip and gently wipe the glass in a circular motion, and then dry it with the other dry end of the Q-tip.
If you have a dirty sensor, you can take either of these approaches: 1) send it to our Repair Center for a professional clean, or 2) purchase an at-home sensor cleaning kit available HERE. Our Repair Center also offers preventive maintenance on all 35mm, digital SLR cameras & camcorders. The preventive maintenance includes: cleaning inside & outside of the body, replacing light seals, checking all functions (including meter, lens, circuitry, etc.) make any minor calibrations, & we will also clean the sensor on digital cameras.