Problem with AF point
It's probably dust on the af sensors. They're located in holes in the floor of the camera cavity.
This happened to my 20D a couple of times.
If you have a bulb blower, start by taking the lens off and blowing under the front edge of the mirror with your blower.
If that doesn't work, put the camera into sensor cleaning mode and you will be able to see the diamond pattern of holes in the bottom of the interior of the camera.
Give them a blow with the bulb blower. That has always fixed the problem for me. Some people report needing to attach a bent drinking straw to their blower to direct the air straight into the holes.
Don't use canned compressed air.
|Fangorn Forest by cand1d|
|Yosemite Falls with Moonbow by Jonathan Shapiro|
from Best Landscape of the Week 4