I'm trying to download pictures from my Nikon

Is the camera recognised by the pc as a usb 'drive'.?? If not try another cable. If another cable does not help, then the fault could be with the camera itself.
Try inserting the card in a separate card reader attached to a pc - if you can see the latest pics taken, then the camera is taking pics ok - and we are back to the transfer problem.

If you can see the pics in your 'my computer' then you can copy or move them onto your pc's hard drive. The fault then could be your Nikon transfer program. Try reloading the software as it may be corrupted on your pc..