I have a Samsung Note 4. I initially thought my 3 year old messed my phone up.....but I think it may be something more technical. I have downloaded about 287MB of music which was visible in all my music players. Recently the music has disappeared. It is on the device\emulated\0\music and visible in a file explorer app and playable from there. I have downloaded next music and pointed it to the folder to scan, and it says there are no music files.
My gallery also lost all my folders e.g whatsapp images, wifidirect images etc. I am now using quickpic which works like how my gallery used to.
Your help is appreciated