Karen,
Sorry if this sounds like a stupid question, but you don't say if you've tried this:
A) Have you tried another card in the camera? Doesn't matter if it works in your computer, sometimes it will work in your computer, or even in another camera, and not in your problem camera.
B) If there isn't anything on the card, i.e., you won't loose anything important, reformat the card (In your camera). If there are photos on it you don't want to loose, move them to your computer, put the card back in the camera, and reformat the card.
**Please make sure there is no software or firmware on the card that may have been downloaded. I lost some stuff once, by just not thinking before I reformatted a card 
. On a side note, it is a good idea to reformat your cards from time to time anyway.**
Sometimes the "read" or "boot" sector is good, so the camera won't give you an error message, but the storage area will have corrupt sectors and therefor won't allow recording.
Please let us know what all you have tried, and maybe someone can help, or has had the same problem.
-FD
