When I open Photoshop (previously OK) I now get a dialog saying to run installer. Where is installer?

I have CC on a MacBook Pro Retina been OK for a year, says it’s all up to date but when I launch Photoshop a dialog says “One or more files in the Adobe Application Support Folder, necessary for running Photoshop, are missing. Please run the Photoshop installer and re-install Photoshop.”  Where is this installer? And why is this?