My SD card isn't recognised anymore in my PC!?

I used to load my digi cam pics direct to my computer by slotting the SD card into the front of the tower, finding the removable storage in 'My Computer' and clicking on it but when i plugged in the SD card a few days ago it doesn't seem to be recognising it's there. The light by the slot comes on to indicate it recognises something is present but where there used to be a removeable drive showing (H, i think) in 'My Computer' it is now only showing my DVD-RW drive. I have an Advent desktop and am running Windows XP. It was working fine 'til the other day and as far as i am aware i haven't changed anything. Is this a common prob? Can anyone help?