iTunes no longer shows song selection menu for sync to iPhone

For years, I have sync'd my iPhone song library (mp3's) by plugging in my iPhone to the Mac's USB port. And for years, iTunes recognizes my iPhone and - until recently - showed me a menu with my music library (on the Mac), where I can select / deselect artists, albums, etc. to sync to the phone. It was perfect - until recently. The Mac still recognizes the iPhone, and I can click the <sync> function from the little iPhone icon, BUT the familiar & useful music selection menu no longer shows up. I have not updated OS recently on either device. What gives?