It depends on the cause.
Have you tried fixing it yet?
First make sure the card is physically inserted. Pull it out, put it back in. Battery pull. You know the basic troubleshooting type of stuff.
Options>Memory>menu button> Intall Media Card/Remove Media card (this toggles depending on what state your in)
Other memory card problem possible fixes:
Just missing stuff? When you open the media and hit explore.Goto each folder giving problems, show hidden files, and delete bb thumbs file, do a batt pull.
Options>Memory>menu button> Format Card (will erase it completely so last resort if you have stuff not backed up you want to recover, anytime you want to use a new memory card you should do this before using it)