I can't upload pictures from my memory card to my PC.?

I have sandisk memory card and i'm using casio exilim camera. the pictures shows up on my camera but when I try to put the memory card on my pc. its says "this folder is empty". Before, when i put the memory card on my pc the pictures shows up but this time, it doesnt even if i use my camera's usb cable. WHAT should i do??? please help me.