CF Card Full but no images on it ? Help Please

Hi

I have a Canon 7d and used four 4GB Sandisk Ultra cards.

While using the camera and cards last night, an error message showed up on the camera saying that the card was full.  Pressing the 'playback' button on the camera showed a screen saying 'no images on card'

When I plugged the card into the computer today it also shows there are no images on the card, but also show that the card is full but it has nothing on it.

Does anybody know what causes this problem and if there is anyway i am able to format the card.

Thanks

Ed