it may be infected by a virus or it may be locked
if its infected then format using a card reader in a pc
or if its locked, then go to getjar.com and download FEXPLORER and after installing open it and go to this path
c:/system
and find a file with name '____store'
(if u r using a mmc card then its displayed as mmcstore and if micro or mini sd cards, i dont know bt it will be having half part of the file name as 'store')
and delete it and reboot ur mobile