First, make sure you're not in manual focus mode, since that would disable any auto-focusing, and then flip to manual and back to auto. If that's not working, you may just have bad conditions for the auto-focuser. You could try using the focus hold or manual focus.
According to the user's manual, it's probably waiting for the focus to settle. If this isn't happening automatically when you press down on the shutter button gently, you may have to use manual focus (there should be a switch on the side of the camera).