You have to do what the error message says, run a disk error checking utility on the SD card. Find an adapter or something that will allow you to connect the SD card to your computer via USB and then run Windows built-in disk error checking utility on the SD card and then you should be good to go. Your files are still on the card, your phone just wont read the card until you run the error checking utility on it. This happened to me last week I believe. I forgot to "unmount" my phone from my computer after putting files on my SD card (or for Windows users, "unmount" is the same as "safely remove hardware"). Check for Disk Errors in Windows XP
There's some instructions.