Please check the data sheet of your card.
There are cards on the market which do NOT conform to the SDHC standard.
It certainly would have been helpful if you had posted the make and model...
Can you still read the card on your PC?
Are the files still there or gone after the card has been unplugged from the Sansa?
Does the card work if you format with your OS?
If it is a SDHC, which speed class is it?
If it is Class 6, please test another card with Class 4 or Class 2.