Nikon D3000 'lens not attached'?

Hi, I'm sure this has been asked thousands of times, but there are so many answer of no relevance, I thought I would start from scratch.

I have just bought a Nikon D3000 and the sigma 24-70mm lens. This isn't for myself and I have no clue about cameras, so any help would be appreciated.

It says 'lens not attached' when in anything other than manual mode.

When in manual mode, i take photos but they are black.

Anyone had this problem and found the solution? Am I using entirely the wrong lens?

Saying this, I have just taken it off automatic flash and have clear pictures. If however, anyone knows if there's a solution to it showing 'lens not attached' in other modes that would be appreciated. It says attach a CPU lens when automatic flash in on and you switch it manual so maybe this is a hint I don't understand.

Nikon doesn't shy away from the fact that they won't guarantee compatibility with third party products… They want you to buy Nikon manufactured kit… And just love experiences like yours.

I would take the camera and lens to an actual camera store and ask one of the staff to help… They won't mind it's part of their job to offer decent advice… You didn't mention the actual model of lens the sigma is… Nikon cameras can accept a variety of lenses… The D3000/5000 series need lenses with electronic contacts to communicate with the lens and the lens needs to have a built in focus motor to autofocus.

What i'm saying is… I doubt the camera is faulty. You have probably just picked up a lens that isn't compatible.