In normal "one shot" mode it will not take a photo until it can confirm that it has focused on a subject. Switch the lens to manual focus (just as a test) to see if it will "take" a photo.
If it does take a photo, then it means the issue is more likely that your lens auto-focus has a problem. (It confirms that the reason the camera wouldn't take the shot is because it couldn't confirm focus.)
"Why" a lens might have a focus problem doesn't necessarily indicate a problem with the lens. If there isn't enough light, this would be normal. If there's lots of light (mid-day, outside, in the sun, etc.) then it might indicate a failure in the lens.
If you have more than one lens, you can swap to another lens to see if the camera still has an issue.
The camera has two independent focus systems... one is used when you use the viewfinder (put your eye to the viewfinder to look through the lens when taking the shot ... in other words the "normal" way) the other is used when you switch to "live view" mode (using the LCD screen to compose the shot.)
It is extremely rare to actually have a genuine problem with the focus sensors on the camera. The more likely issue is a problem with the lens (that's where the moving parts are ... more likely to fail).
If it is actually a lens failure, then you have to decide if it's better to send the lens in for repair... or just replace the lens. Labor (hours spent correcting the problem) are expensive. So for entry level lenses it is usually cheaper to replace them. For very high end lenses it is usually cheaper to repair them.
Tim Campbell
5D III, 5D IV, 60Da
