Why are half of my photos missing in my gallery on a Galaxy S5 Android

[AC Question]

I go into my gallery and look half of my photos are missing don't know where they went I didn't delete them or download them on a memory card so somebody help me please

 

[ManiacJoe]

Galaxy S5
Which version of android?
Which carrier?
Which picture viewing app?

Using a file browser app, are the "missing" pictures still in the expected folders?