Assuming that camera is like other cameras, it should have two primary AF modes. Moving subject mode (AI Servo (called by others AF-C)) and still subject mode (One Shot (Called by others AF-S)).
In still subject mode, the camera is programed so that when you press the shutter release, it focuses and waits until it has achieved focus before actually snapping the photo.
If, for whatever reason, you can't achieve focus (too dark, no contrast) then the camera simply won't take the photo...in that mode. If you switch to a different mode, like moving subject mode, you should be able to snap a photo whenever you want....even if it's out of focus. Or you might be able to turn the focus off (manual focus).