You have probably corrupted your card. If you do not remove the card from your camera or computer properly this can happen... and it is a common occurance.
Try formatting the card in the computer.
If that works, run ChkDsk on the card if you know how.
And then reformat the card in your camera to set the allocation block to 32kb.
Note to Jini; you should reformat your card in the camera. When you format the card in a Mac, it defaults to a 4kb allocation block - which is not the most efficient. While the card will work in the camera, it will be slower than it is capable of.
Cameras typically format cards using a 32kb block, which is why if you reformat the card in the camera, it will be more efficient.
You did the right thing though - you formatted the card in a Mac to get it to work, but then take the extra step to format it again in the camera to reset the allocation block.
If you have a windows computer, it can be set up to format the card with a 32kb block size so you would not have to reformat it in your camera in that case. But there is no easy way to format 32kb in a Mac without 3rd party software. This is one time a Mac is brain-dead vs. windows.