Sounds like the SD card is corrupt.
I would suggest backing everything up to your computer (apps/roms/etc)..
formatting the SD card with FAT16 filesystem (don't do quick format, do the full format).
Then recopy some of your stuff back to the SD card and test it out. I've had this happen with a cheaper 2GB SD card that l used to use and after l reformatted it and copied everything back over it worked fine.
If you want to be anal about it you can copy over your files little by little but l just dumped everything back onto the SD card and it worked after that.
Regards,