The likely explanation here is that the card has a hardware fault in the SD card which will prevent any attempt to read from it. If this is the case, your data is likely to be permanently lost.
This is not necessarily 100% certain. It could be filesystem corruption as suggested by mattdm, as that can sometimes manifest itself as the drive not showing in Windows. One way to tell in Windows is to open Disk Management: in the start menu search for "Disk Management" and the option "Create and format hard disk partitions" will come up. If you see a listing for your SD card then it's likely that the card is readable, but has filesystem corruption, in which case go ahead and use PhotoRec as suggested elsewhere. It's worth trying PhotoRec anyway.
However, there is still a real chance that it's a hardware fault with the SD card.