I've had a Mac OS X Yosemite since January and have been using Creative Cloud for a few months with no problem. But I've been away for a few weeks and on return today Photoshop will not initialise, although Lightroom goes ahead just fine. I have tried to find help and can only find things related to CS6 (which I never had) or to resetting Photoshop Preferences to the factory default - but as I can't even get Photoshop to start, this seems impossible. I am not aware of having changed anything. I got a Photoshop update today, but that was just about extending the Camera Raw functions. It hasn't improved anything.
I have no idea what else to do, and am not too familiar with the Mac setup. Please advise.