If it used to work in both the phone and the computer, and works in neither, now, then the file system is probably corrupt. Formatting the card should fix this, but you'll lose the data on it. Best Buy can probably recover the data, but it won't be free.
As an aside, for a card (or any flash memory) that becomes corrupt for no apparent reason, even after formatting it, don't put anything important on it for a while. I had a flash drive that corrupted on me, I formatted it and used it for a few days, and the data corrupted again. Tried it again, same result, so it was relegated to garbage duty. Basically, the corruption could be a software issue (issue caused by software when reading/writing from the card) or an issue with the 'hardware' of the sd card.
