|It's very likely that the memory card is faulty and is no longer usable. |
You can try reading the card with a card reader which plugs into a USB port, that's the way most people transfer pics from a camera these days. Transferring pics by connecting the camera to the PC is unnecessarily fiddly and I dropped my camera once trying to do it that way.
If the card reader shows no pictures on the card, try recovery software, and if that doesn't find them I'm afraid you've lost them and you shouldn't use that card any more.
Recovery software here: http://pcsupport.about.com/od/filer...