Why does my flash drive not work?

I have SanDisk MobileMate SD card compatible flash drive. When put the SD card in my phone, everything works, and i am able to acces my music and photos without a problem. However, when i put the SD card into the flash drive and plug it into the usb drive of my computer, it does not show up. This has never happened before. A couple weeks ago i was able to use this hardware without a problem. Now all of a sudden it isnt working. When i plug the flash drive in alone, nothing happens; when i put the SD card into the flash disk, the only thing that changes is that a blue light illuminates the back of the flash drive. Can anyone please tell me why i am not able to acces my SD card through my flash drive on my computer.