I'm hoping you could help me with my Canon EOS 1100D. If I use the manual focus mode, then everything works properly. However, when I try to use the automatic focus mode, then it'll just refuse to take a picture. I've tried it with the Tv, Av, and Manual, Automatic, and Creative Automatic, but nothing seems to work when I try to use the automatic focus. I think the automatic focus just randomly stopped working.
With some of the cheaper lenses, trying to focus manually while the lens is set to AF can jam the autofocus motor. Sometimes this effectively destroys the lens, but sometimes it can be corrrected my manually un-jamming it. There are several threads in the forum that explain how to do it; read at least one of them before you consider trying it.