Is it not locking focus, or are the actual lens elements not moving?
When it tries to focus, you should be able to tell if the physical focus is at least changing. If you can hear things moving and the actual focus of the lens is not changing at all, my guess would be that somehow the motor has become disconnected from the focus group, and you'll have to send it back to Canon.
If it's trying to focus, and just not confirming or locking focus, I'd be curious what you're shooting. Can you add more light, or focus on something higher-contrast? What focusing mode on the camera are you using? Is there another camera you can borrow from someone to test it? If you throw on a speedlite, does the AF assist beam help at all?
Just my initial thoughts on how to better diagnose the matter...