Try the following and see if any work for you.
Option 1.
Go to "Settings" and then "Storage." Unmount the SD card and then Remount it (without having to actually take the SD Card out).
Option 2.
Turn off the phone, take out the SD card from the phone and then put it back into the phone.
Option 3.
Should neither work, back up all files that are on the card to a computer and then got into "Settings" and then choose "Storage." Now you want to choose "Format SD card". Once it has been formatted, you can put the files back on it.