Try these two methods:
Method 1. Reset Gallery to make SD card photos/videos show up in Gallery
1. Go to File Manager > Android > Data;
2. Find com.android.gallery3d file folder and delete this folder both on your internal or external card;
3. Go to Settings > Apps > Application Manager;
4. Search for Gallery and open Gallery > Click on Clear Data;
5. Switch off Android phone and switch on.
Method 2. Delete .NOMEDIA file to make SD card photos/videos show up in Gallery
1. Go to the respective Folders where images/videos are stored on SD card;
2. Click on Settings > Tap Show hidden files;
3. If there is .NOMEDIA file in the folder, delete it;
4. Switch off and switch on Android phone.
There you shall be able to view SD card photos, videos in Gallery and use other files on your Android phone again.