First, here's a
copy of your manual if you need a copy to check while away from home - or if you lost yours. The info below and page references are from the link provided above.
Autofocus operation depends on general lighting, subject contrast and
detail, and other technical factors. In rare situations where autofocus
(and manual focus with Electronic Rangefinder) is not possible, ">
First, make sure the camera is in AF (auto focus) mode. You find the switch on the front of the body, near the bottom to the right of the lens (looking at the front of the camera). Make sure the pointer is on AF, not M (Manual). This is detailed on Pg 31.
Second, make sure the "solid O" is displayed in the viewfinder. This indicates the camera has locked focused successfully. If the "" and "
1) Very dark subject. Solution: Focus manually with clear matte field, or for Single Servo AF, focus on another brighter subject located at same distance, lock focus, then recompose (pp. 36-37). Or, use a Nikon AFSpeedlight (SB-28, SB-27, SB-26, SB-25, SB-23, SB-22s, SB-22 or SB-20) to perform autofocus with Speedlight's AF illuminator., 2) low-contrast subject and 3) Strongly backlit subject or bright subject with shiny surface such as silver or aluminum, or scene in which there is a pronounced difference in brightness.
2) Low-contrast subject. Focus manually with clear matte field, or for Single Servo AF, focus on another subject at same distance but with more contrast, lock focus, then recompose (pp. 36-37).
3) Strongly backlit subject or bright subject with shiny surface such as silver or aluminum, or scene in which there is a pronounced difference in brightness. Focus manually with clear matte field.
Check the mode you're in. Generally, "Program Mode" will provide "shootable" settings 99.9% of the time. Creative control in this mode however is lost. Try switching to one of the other Modes to get some creative control back. Vari Program allows you to change shutter speed or aperture and automatically make other adjustments to allow the shutter to release. Similar or greater creative control is available when the Aperture Priority, Shutter Priority or full Manual exposure modes are selected. You may be able to compensate for overly bright subjects by decreasing the aperture size (increasing the f number), or for dark subjects by increasing the aperture size (decreasing the f number). You may need to change Exposure Mode to gain control of aperture settings. See Page 46.
I hope this helps - and good luck!