check and see if it's write protected on a PC. if it is, your card is going bad and you'll want to copy everything off of it and get a new card. if it's not, try turning the phone off, unseating and reseating it, then turn the phone back on.
this happened to me a while back, and shortly afterwards my card did go bad. i had replaced it with a class 10, but i gave it to the wife, and her Dinc2 kept unmounting it and eventually was unable to write to it.