Auto Focus Inconveniently Stops Working

EOS 40D, happened last night while shooting a time-critical event: autofocus just stopped working and I was unable to shoot. Had a brain cramp, so I grabbed my backup Rebel to finish up. Found today that the 40D will shoot fine in manual focus.
Had this happen 6-8 months ago, and I was able to clear it up (it seemed) by removing the lens and cleaning the camera body contact pins and lens contacts, but it changes nothing now. Doesn't make any difference what lens I put on (tried 3 last night during my frenzy!) - none of them worked, so appears the issue is in the camera, not the lens. I've done a "clear all camera settings" just in case I inadvertently did something crazy, but that changes nothing.
We're many miles from a Canon repair facility; I'd like to solve this if I can. Any ideas for me? Thx!

What lens?

Nick

Do you have a grip attached? If so remove it and try again. Failing that, take both batteries out and leave for 30mins before refitting. This will reboot the electrickery insde the camera.

Lens: Canon EF 28-135 IS. However, I pulled it, put on another Canon (kit 18-55 from the Rebel, and a Sigma 24mm 1:1.8 EXDG, and neither lens changed the response.
Yes, I do have a battery grip. I just pulled it, and will leave out all night and try again tomorrow - will post an update then w/status. Thanks for the idea.

Remember the tiny battery for the camera settings.

You mean, pull the little watch battery, too, right?

Yeah, remove that too. That will let the camera completely clear itself and go back to all factory settings.

Nick

I have seen half a dozen cases like this with several 40Ds. Also occurred with 30D. In the viewfinder if the green dot is blinking when halfpress the shutter button, that means no focus is achieved hence prevents you from taking a picture. This could be a result of dirty focus screen.
If the green dot is solid and a focus beep is heard but still can't take picture, it's likely that the shutter button connector is dirty. Before sending the camera to Canon, there's a cheap method you can try and see if it helps. It may (or may not) save you money. Check it out here http://www.youtube.com​/watch?v=tB_gSqyidI0

Thanks, all. I'm out of here for the evening. All batteries pulled, will report back in the morning.

If you do the alcohol thing, be sure to keep the camera tilted as in the video, and do not ever just lay the camera upside down level. The alcohol will most likely get into the top LCD screen layers and bubble them. It has happened before.

Success! Grip and small memory battery left out all night, and it's working fine this morning. Now, the real question: "why does this happen?", followed by "Is there a better immediate action fix to permit shooting?" Any exact idea of how long the batteries must be out to kill the camera's memory? 5 minutes might be workable, 30 won't solve the problem on site. Thanks to all who helped me!

If camera is turned on when you rip the battery grip, any reserves for driving the processors will drain almost instantly. If camera is in power-save mode, the spare power in capacitors will keep the processor supplied for quite some time, but probably 5-10 min is enough.

If the issue requires a reset of the configuration memory, it's way worse because that memory could - potentially - survive hours without the button cell. The power consumption is almost zero, so if there is a capacitor inside the camera it will happily feed the config memory.

If problem happens again, an initial test is to keep camera powered (display on) and pop the batteries from the grip. Then just wait 2-3 min and put in the batteries. You could potentially try to press the shutter button without the batteries - if processor did go down in power-save fast enough, you might possibly trick it with this wake-up request, to make sure it quickly drains spare power from capacitors.