I have been using the 17-40 mm L on my 20D for almost an year now. Since yesterday the lens has stopped autofocusing. This is really strange, since nothing has happened that can explain this behaviour. The lens hasn't banged anywhere or fallen down, the contacts are still shining. Plus I have checked that the problem is with the lens, since another lens (EF 50mm) works very fine with the 20D. And the 17-40 mm exhibits the same problem with my older EOS 300V.
Now this is the problem: In the AF mode, when the shutter button is half-pressed, nothing happens. The lens has simply stopped autofocusing. Of course I have checked that the lens AF button is selected. Otherwise the lens is okay, e.g. if I manually focus with half-pressed shutter, the camera beeps when the focus is achieved.
Could anyone explain what might be the case? Is it sth to do with the USM mechanism? Is the lens motor dead? I really hope someone can understand and locate the problem and offer some advice.