I was listening to iTunes Radio yesterday - US iPhone, US Apple ID - after launching it from the 'Radio' tab in the Music app (mentioned here). Today that tab disappeared. What happened?
iTunes Radio runs as a separate app. If it's running in the background, the tab may disappear from Music. Double-tap the home button to see background tasks, swipe left to find the running Radio app. Swipe up to kill it, if needed, and the tab in Music returns.