I put my memory card in the card reader and then the green light come on then nothiing?

My computer runs fine my memory card works fine in my camera i put the card in the card reader worked fine. went out took some more pictures came back an hour later put the card in again the green light came on but no windows promt window I went through my computer and clicked each removeable disk drive and its staing nothing is there. It just worked an hour ago what happened? my sd card for my ds works fine in the slot above the scan disk memory chip for my camera . The card is a scan disk memory pro duo from magic gate for a sony cybershot camera.