Has this card worked before? Is your card a SDHC (secure digital high capacity) and your card reader only SD compatible? SDHC cards can only be read by a SDHC card reader. SD cards on the other hand can be read in either SD or SDHC card readers. The difference between the two cards is storage capacity. SD cards are up to 4GB and SDHC is 4GB and higher. Because of the larger storage in SDHC cards, they can write quicker than SD cards.
Is your card reader built into your computer? If so, you may want to consider purchasing an external card reader and testing your card(s). They are pretty cheap (less than $15). That way you could pinpoint on whether it’s a hardware issue or a corrupted SD/SDHC card.