Is there a chance that you have switched to RAW setting instead of jpeg? If this is the case your PC may not be able to see them, (although it should if you have the relevant Canon software on your PC) but the camera will show the little jpeg preview on the rear screen.
matey i think you may have sussed it i am on raw so i'll have a play
& see if i can get it to work. let you know how i go on cheers