Well, your SD card could have died.
Originally Posted by Poehring
Have you tried changing USB ports? It sounds stupid, but it could mount under another usb.
Found this on another forum. "This has worked for a few people i've told that had this issue, download anycut in the marketplace, then create a shortcut, activity, SD Card. Then open the shortcut that it creates, should let you manually set to mass storage."
And finally, something that worked for a user like you: if you are still having this issue, get a different "data transferring USB cord" not a "charger cable".
the difference is the following: A "data cable" is one such as from a camera, the original usb from g1, an external HDD reader, a usb that comes with purpose of transferring information. A "charging cable" is one such as the ones that comes for the play station 3 controlers for wireless, or any other cable that plugs somehwere for power, such as a multi outlet usb charger cable (the sell those on g1 stores, they wont work for data)."