Canon Autofocus problem

SOURCE: e-500 autofocus problem

mine will do this at random and I simply have to remove the lens and re-install to correct it and it will focus from then on.

SOURCE: My Canon digital rebel will not autofocus correctly.

I had this problem awhile ago - you must have accidently adjusted the focal area - it is sooooo easy to do without realising it. Here is what I was told:
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Most likely, you have changed the AF point, which only takes effect in the creative modes (Manual, Av, Tv, P, etc)

There should be a button on your camera that has a grid like pattern above it, like
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- - - - -
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or so. press that, look through the viewfinder, and move the shutter or rear wheel, you should see the different AF points like up in red, and as you turn different ones will light up, and eventually all of them. also, you can look on the screen on top (or back, if its a rebel) and it will show you which ones are selected while in that mode, instead of looking through the viewfinder.

SOURCE: nikon d40 autofocus

Out of the box, I couldn't get a center focus shot. Out of desperation I reset the camera (push info and magnify buttons at the same time). That solved the problem.

SOURCE: Canon digital camera will not focus

Check your diopter adjustment. It should be a small wheel on the upper right of the viewfinder. It is designed so that people who normally wear eyeglasses can use the viewfinder without their glasses, but it can occasionally get bumped. If the diopter adjustment is way off, it is like looking through someone else's eyeglasses!

SOURCE: Canon 300d autofocus problem

Sound like the secondary autofocus mirror may be loose, gently move the main mirror up after removing the lens to have a look. If this is the case the camera needs to be repaired