4.4 kitkat - Photos in the Camera folder in the Gallery were deleted automatically in my Android phone

I am wondering about how it is possible that all the pictures in the Camera folder in the Gallery were deleted without my knowledge where I am the only user of my Android phone. I don't remember exactly how many pictures were there in the camera folder that I have been capturing for almost six months. But they were there till the last night and only this morning I was shocked to have discovered the deletion.Although I know there are many software through which I can recover my photos,I want to know the possible causes behind.Is it due to the maximum size of photos that can be kept in the camera folder in the Gallery,surpassing which may trigger automatic deletion of the photos or just something else? If so,then how do I manage the size or number of photos? My phone is not rooted. Any advice is welcome!

The screenshots of my device memory is:

^{Screenshot - Storage (Click to enlarge)}