first go to menu.settings.sd&phone storage does the third option down say "mount sd card"
if so press that and see if that fixes it.
if not remove memory card, place back into phone and see if it works
if not remove the memory card and plug it in to a card reader on a pc and see if it reads it. back up contents and format it. put contents back on, put back in phone.
if all else fails try a different memory card