Is there enough light in your scene to achieve auto focus? If not, you can try using the focus assist light, (Custom setting A7) or using a flash attachment that has focus assist.
Are both the lens and camera set to auto, (lever on front of camera set to "A")?
Did you try focusing with the AE-L/AF-L button on the back of camera,(if custom setting F5 has been set to "AF/ON")?
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