Trouble with trying to transfer images?

I have a Nikon D5100 and have previously transferred images successfully to my PC for printing. Unfortunately, for the past year or so ill health and visits to hospital (several !) have prevented my using the camera at all until a few days ago. I find that now I can't transfer images at all. The instruction book tells me to connect the USB cable (with the computer switched on) then switch the camera on (same for transferring by using the SD card in the slot in the PC), then select from the dialogue box. Trouble is, no dialogue box appears and if I click on "Nikon Transfer 2) and then on "Source", there is no sign of any device in use ! Am I doing something stupid ? I certainly had no trouble transferring images a year ago. Incidentally, to forestall a possible suggestion, the instruction tells me to ensure the camara battery is fully charged, and it is. Any ideas would be welcome, so thanks in advance to all those who try to help.