Since you did not say, I will assume that you are using One Shot Focus mode. One Shot mode requires that the camera lock focus on a subject before it will take a photograph. By default, the AF point in the viewfinder will turn red, flash, and the camera will emit a beep. You cause the camera to focus by gently half-pressing the shutter. Barring some physical problem that requires a repair, there are some checks that you need to make.
There are any one of several reasons why the camera cannot focus when you press the shutter. You can be too close to the subject. You are two close to the subject. There is not enough contrast on the subject, for instance you will have a difficult time focusing on a plain blank wall.
It is also possible to reprogram the shutter button so that the camera does not refocus when you press it. Then, one of the buttons on the rear panel, typically [AF-ON], is reprogrammed to activate the AF system. Users have been known to experiment with this feature, and then forget that they have done so. Resetting to factory defaults does not clear the custom programming. YOU must go into the custom programming menu for the buttons, to clear or reset the custom button programming.
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"The right mouse button is your friend."
