I just received my Nikon D3100 and for the first few photos it worked beautifully, but now when I take photos it only shows approximately the top 3mm. I put on both lenses and removed the caps but still the same thing is happening. I cannot afford to fix it after only one day of use. It looks almost as if a black cover is covering most of the picture.
It could be a problem with your memory card. Some cameras allow you to take a picture without a memory card and review it for a short time. Try to see if you can do it and see if the image looks ok. Then try it with your memory card on. If your image looks corrupted then the problem is with the memory card. If not you can atleast rule out the memory card as the cause.
It would also be helpful to have a sample picture.
My memory card (Verbatim) on my new D3100 went corrupted within a month of purchase and I lost some precious 250 photos due to this. If your memory card is Verbatim ( which comes free with Nikon D3100), I would suggest buying a backup memory card.
The first step to do when you insert a memory card into a Nikon D3100 (or any other DSLR) is to format the card using the camera itself. This step is to make sure that the file system on the memory card is compatible with the camera.
It is also recommended to reformat the memory card each time you have completed transfer of pictures to your laptop or computer.