My SD card reader shows no images but says it is full and there's nothing on the sd card?

I have a nikon d3100 and it seems to have broken all my SD cards.one i put it in the camera and then take them out to get the photos off, when i put it back in it says the card is full but i can't find any images. I'm not to sure if it is my camera or my SD card readers. But the same thing has happened to all 3 of my cards. I tried using them in another camera and the same thing happened. What do i do?

I have had the same problem my SD cards and reader, and not just the ones from my camera (Not a Nikon but Fuji) connecting the camera to the PC via USB is painfully slow to copy over but more often than not if I take SD card out and put it in the reader I get the same problem as you and also with my Dell Axim when changing software or updating software. Over the past few months I have worked out that it has been a software problem and not hardware, two months ago I wiped my computer and reinstalled XP then the drivers and software for the camera and Microsoft active sync for the Axim and all was well and good took an SD card that has pictures on put it in the reader and Nothing! So I found the drivers for the card reader and it worked OK again, for about 1 day and then back to the original problem (card full but no pictures) I have a laptop running Linux Mint connected the reader to it no problem I can see the card and the pictures, I also have a MAC an that has no problem either, plugged the reader back in to the PC and "New hardware found" comes up put the cd in the drive for the drivers and all is well again, for a while at least and the it stops reading them again so I assume that it is a software problem
So now I just keep the pictures on the SD cards and when they are full I buy a new one (8Gb from a good company £6) as the cards are "non-volatile" they are about the safest place to store my treasured pictures (safer than CD/DVD, hard drive or on line as my partner found out when the on line company went bust and she lost over 1000 pictures) if I want a picture for what ever reason I put the card in the Camera, connect it to the PC via USB and make a cup of tea while it copies the pictures over. I know this is not really helping but at least you know it is not just you.
If you can still view the pictures on the screen of your Nikon the it is more than likely that the card is OK, these cards are pretty robust things after all, try getting the latest software for your card reader and camera
Hope I have helped