Assuming your SD card is not damaged or corrupted, it seems that media scanner has not picked up your media!
You could try an app like Force Media Scan to force it to run. NOTE: I have not tested this app. There are many apps to do this this one has a widget for it (also not tested by me). Some applications like PowerAmp will automatically scan for media when first booted also.
Alternatively you could try to update or re-install Google Play Services (though this may cause issues with other applications). If you are a bit more techie you could try running media scanner via commandline.
Finally I found this article where the author says none of the apps worked, but the following did (note it was on a Galaxy S3, but I don't know what device you have):
- Turn off you phone and pull out the memory card.
- Insert the memory card in a memory card reader. If your desktop or laptop has an MMC slot, it will do.
- Wait till you can see the Drive of the memory card (Drive E, F, G, H, I, J, for example).
- Do not open the card in PC.
- Now click on the “Start” (Windows) button on your PC and type “cmd”. Hit the Enter key and you will see a command box.
- Now type the following command at the cursor blink:
chkdsk x: /r
- Here “x” is the value/name of the memory card Drive. In my case it was H, so I typed
chkdsk h: /r
- Now hit the Enter key on your keyboard and wait till Windows checks your card and fixes it.
- When it is finished, safely remove your card and insert it into the phone.
Reboot the phone and now you should be able to see your media files on your phone’s apps.