If you don't care about the data on the card you can reformat it. If you do, you should get a cable (didn't one come with your phone?) and see if you can read it from a PC.
Have you tried a battery pull and removing/re-inserting the card yet? If so and that didn't help...
If there is anything that you hope to save on the card, like some pictures, you'll have to get a usb cable and copy the contents to your pc hard drive, then format the card (from the bb...options>memory>format>media card), then copy the data from the pc back to the card. If you still can't read the card data when you first plug into the pc, you'll have to get one of those card readers to plug it into and try to copy to the pc that way.
This can happen occasionally and it's the reason that you should periodically make a copy of the card to your pc whenever you add something to it that would be painful to lose.
