There are a few things you can try:
1. Go into the camera menu and reset it to the default settings. Also go into the Custom Functions and reset those to defaults. It's possible someone has set up Back Button Focusing or some other settng, which might fool you into thinking the lens isn't able to focus. By resetting to the defaults, you can be certain AF is turned on in the camera.
2. Clean the lens-to-camera contacts. To do this, remove the lens from the camera. Lightly dampen a clean rag with isopropyl alcohol (aka, "rubbing alcohol") and use that to carefully wipe the gold electronic contacts on the rear of the lens. Be careful to keep off the optics of the lens. Note: I don't recommend using a pencil eraser, which is sometimes suggested. Often the problem with these contacts is oils... finger oils or oil from lubricants in the camera or lens... and erasers are made from vegetable oil, so aren't really good removing it. Widely available and cheap rubbing alcohol will do the job.
3. While you have the lens off, also check the gold "pins" just inside the front of the camera, that correspond with the electronic contacts on the lens. Clean them the same way, being very careful not to touch the mirror or anything else in there. Also lightly press on each of the pins, they are spring-loaded and should pop back into place smartly. If one is "sticky" from oil or dirt or whatever it might not be protruding enough to properly make contact.
4. Operate the lens' AF on/off switch a number of times. If never or very rarely operated, these switches do occasionally get oxidization inside, that stops them from making proper contact and interrupts the flow of power to the AF motor in the lens. Most of these switches are "self-cleaning" when operated. Just turn on and off a dozen or more times. AF just might start working normally again.
5. If the AF still doesn't work, take camera and lens to a store that sells Canon lenses and ask to try one of their lenses on the camera. If it autofocuses okay, then you know it's the lens on your camera that's at fault. On the other hand, if the borrowed lens doesn't AF, then you know it's a problem with the camera. I agree with other posts, it's far more likely a problem with the lens than with the camera, but you'll never know for certain until you test with a "known good" lens.
6. It's unlikely to help, but you also might download the latest firmware from the Canon website and install it on the camera (or reinstall it, if it's already up to date). It also is not likely, but might help to "reboot" the camera (however, this procedure is not possible with all models). Sometimes firmware updates and rebooting the camera clear mis-instructions that are stuck in the camera's memory... sort of like rebootiing a computer to clear the memory or reinstalling software that's gotten corrupted. After all, today's cameras are essentially computers, too.
All the above cost little or nothing other than a little of your time to try. If there has been a failure and it's just the lens, it might be a relatively inexpensive repair such as a ribbon cable that's come loose. Or it could be a more expensive issue such as a failed AF motor in the lens. Depending upon the lens, it might be cheaper to just replace it. If the problem is with the camera, repairs might be pricey (unless camera is still in warranty). If it's fault with lens or camera, you'll have to get it looked at to get an estimate of cost to repair.
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
