I'm guessing your on ICS, I got that problem all the time on ICS and never on GB. Anyway download, install and run this app and your SD card will be refreshed so media should be detected. Everytime you get that problem just run the app.
If that does not work then go to the applications section in settings go to all apps and scroll down till you see media storage or media server (something like that), tap it and clear its cache.
If neither of the above solutions work then backup your SD card, format it and then restore the data.
