I used the card reader with a SD card and it worked great. Then I went to use it again and it will not work now??????

How did the photos get onto the card ? If you copied them onto it yourself from your computer then you need to create a DCIM directory off the root of the card, with the photos underneath it, and the photo filenames need to be exactly 8 characters long (no spaces) plus the file extension i.e. in a similar format as if a camera had created/written them.

If the card has come straight from the camera or it has the above DCIM structure then have you tried a reset to see if it's recognised after the iPad has restarted ? Press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.