Transfering pictures?

What's gone wrong?
Coolpix 8700 Nikon Digital camera which I'm new to, given to me. Worked ok on transfering to PC have been transfering pictures with no trouble for a month and now nothing!.
Rear screen blank when USB lead connected, PC gives audible sound to indicate connection made. But screen shows no transfer menue. The top LCD screen has the normal animated lines in rectangle shape. Can takc,view 4sec view, view manuallly and as slideshow so no probs htere. it's just this Transfering.
I'm wondering as Im new to the camera I'ver pressed a wrong key or something.
Thanks