Weird, but I also though it was gone -- until I saw that it is a different, weird icon now. Look in the same spot for an icon that looks more like a square and triangle. Click on it and you'll get the Airplay functionality. Still -- why did they change it???
All right. Well then, I would only suggest the same solution Des Collins had. Check if you're connected to the same network as your ATV. Maybe your computer falsely logged into an open neighboring network or isn't connected at all.