if reemoving and reinserting the card doesnt help, youll need to have it reformatted....
do this by connecting your phone to the pc and opening your MY COMPUTER
icon > right click REMOVABLE STORAGE/DISK > select FORMAT
**if your computer says something like 'no disk available' try disconnecting the device and select MENU > SETTINGS > SD CARD AND PHONE STORAGE > make sure the check bos for 'USE FOR USB STORAGE
' is checked****
after that, set the 'file systems
' as FAT32 > select START > read the warning > press OK > once complete, disconnect from the pc
and try again!
IF that doesnt work, try to test the sd card in another phone to see if the sd card itself is the problem. if you still have the issue but the sd card works, go ahead and reset the phone.
MENU > SETTINGS > SD CARD AND PHONE STORAGE > FACTORY DATA RESET > RESET PHONE > ERASE EVERYTHING