Lost music on iPhone after upgrading to El Capitan(?)

I went for a run Saturday morning and listened to my running playlist on my iPhone 6s+, upgraded my MacBook Pro on Sunday from OS X 10.8.5 to 10.11.4, and just discovered Monday that all of my music on my iPhone (iOS 9.3.1) is now gone. I did not sync the iPhone with the MacBook after the OS X upgrade...In fact, I always select the option to "Manually Manage" music on my iPhone. I did upgrade iTunes to the latest version after the OS X update.

So what gives? What happened?