I woke up today and my Gallery was showing 0 Image, Video or Audio files. After seeing this thread Images have disappeared from gallery, but still in "My Files" (Samsung Galaxy S6 Edge, 5.0.2) , I plugged in my device to a laptop and deleted a .nomedia file in the root folder of my device. I deleted this file, which fixed the issue and the device rescanned everything automatically and showed all of my files.
My question is why did this file get added to my device? And if I had been unable to access a laptop how do I delete a .nomedia file from within android?