Weird. This loss of all music from my iPhone happened to me today. I opened the Music app on my iPhone 5, it showed my playlists for a couple of seconds, then crashed; when I reopened the Music app, it said there was no content, and that I could get music from the iTunes store. If I search for Music files on my phone - songs that I know were there - nothing appears in the search results.
Context: I guess the last time I used the Music app was last night, playing one Audiobook while going to sleep, using the timer in the Clock app to make it stop playing. There have been minor syncing bugs, but that pretty much goes without saying, doesn't it?
I've now plugged the phone in to my computer to resync; all the music has apparently reverted to "Other", so this syncing process may take a long time, depending on how willing the Other is to disappear without doing a full restore.
I'm not concerned about having lost anything - the stuff is all on my computer, and telling it what to sync is easy - but the resyncing will likely take at least a few hours. However, sometimes I'm away from my computer for a month or so, and if this happened at the airport at the start of a trip I'd be more than a little annoyed!
(I assume there are other threads about this, but this is the first one I found.)