The Nikon D40 is a compact digital single lens reflex camera. It was introduced in 2006. The camera features a 6-megapixel CCD (charge coupled device) sensor and can record images at a rate of 2.5 frames per second. The camera can be used in full manual mode or various program and scene shooting modes. It is compatible with autofocus and manual focus Nikon lenses. The sensor is a DX cropped sensor that requires photographers to apply a 1.5 FOV (field of view) crop.
Remove the battery, memory card and lens from the camera. Digital cameras need power to operate. The Nikon D40 uses a rechargeable lithium-ion EN-EL9 battery. If this battery no longer holds a charge or is empty, the camera won't turn on.
Clean the battery compartment and lens contacts on the camera. Moisten a cotton swab with a small amount of rubbing alcohol. Carefully clean the contacts inside the battery compartment, removing any dirt or grime built up on the contacts. Clean the small metal contacts on the camera lens mount and on the lens itself. Use a microfiber lens cloth to remove any excess moisture from the contacts. Let them dry thoroughly before reattaching the lens.
Clean the contacts on the battery, using a swab moistened with rubbing alcohol, in the same way you cleaned the contacts in the battery compartment.
Replace the lens on the camera, and install an empty memory card and the battery that was removed. Turn on the camera. If the camera still does not turn on, inspect the exposure mode dial on the top of the camera to see if it is stuck between two modes, which could prevent the camera from turning on.
Install a new battery if the previous steps did not result in the camera turning on.
When downloading pictures to a computer, it is recommended to use a memory card reader instead of attaching the Nikon D40 directly to the computer. While the risk is low, there is a possibility of an electrical charge from the computer damaging the camera or battery.