I have a Canon 350D Rebel XT DSLR, one day, when i wanted to use it, the autofocus just doesn't work. The manual is doing good, but when i turn it to autofocus, it just doesn't focus. Sometimes the lcd display shows "busy", or the autofocus indicator just kept on blinking.
Two main possibilities, the AF system in the camera for identifying proper focus may be shot or the AF motor in the lens may be shot. If you have more than one lens, switch lenses and see if you still have a problem. If multiple lenses can't focus, the problem is probably in your camera, possibly the AF sensor is not functioning.
If only the one lens doesn't work, them there is likely a problem with the AF motor in the lens and it would need to be repaired or replaced.
I stumbled over the same problem, my EOS 350D would not autofocus when in Program mode (P), but OK when in Auto mode. It would just focus in and out, and not stop at the right focal point. No matter which lens. My solution, after cleaning contacts and playing with all Custons Fn., was to push the AF point selector button (rear, top right), thus getting a bracket-picture in the LCD. By turning the main dial wheel (behind shutter button), and placing the pointer in the bracket in the middle, the focusing works. Hope this might solve someones problems.
Try giving the electrical contacts on both the lens and the body a gentle cleaning. If either side is dirty you may not be getting a proper connection.