Android - Why can't my phone find pictures or music files suddenly?

Ah, cripes. I don't know what on Earth put this .nomedia file on the root of my SD card. Problem fixed.

This issue has been driving me insane. It's happened previously on Samsung Galaxy S4 and again on my S6 edge.

Issue: Images missing from Gallery app or Music missing from music player app.

Having tried everything from using ES explorer to move files to wiping the cache partition, there's a simple solution:

Plug your device into a PC. Identify the folders that are not showing (i.e., DCIM and Music). RENAME these to something else (such as "DCIM2" and "Music2"). The files will reappear.

I then renamed the folders back to "DCIM" and "Music". It took 48 hours of online searches, learning about the 'emulated' folder etc - but this was the simple solution. I guess it reindexes the folders and makes them be found again by the appropriate apps.

Hope this helps.