Samsung just doesn't seem to play well with microSD cards--at least, from what I can tell from posts on these forums. The card or a file on it might have become corrupt. Unmount the card (in
Settings>Storage), remove it, and insert it into your computer (using a fullsize SD card adapter--very cheap to buy, if you don't have one). See if you can see your files on the computer--if so,
back them up immediately. You could run chkdsk on a Windows PC to see if there is any corruption present:
http://forums.androidcentral.com/am...guide-using-chkdsk-fix-corrupted-sd-card.html
Then consider reformatting the card (the best option is to insert it back into the phone and select the format option, if available). After reformatting the card, transfer your files back on, and see if it works again. If not, you may want to get a new card. But be aware that there's the chance it might get corrupted again--so be sure to backup photos and other important data frequently.