Open the CC desktop app, sign out and back in.
Just out of curiosity, have you uninstalled any older PS versions, leaving the newer ones in place? This can sometimes mess up the registry, leaving orphaned pointers to those older versions. Adobe keeps saying this is not a problem, but I know from experience it can be...
EDIT: Oh, see you're on Mac, so no registry. Still...
I've found the solution! I'm kinda kicking myself as its obvious now. I'm using smart previews so I can remove my original files and speed up my workflow. I've just reattached the external hard drive containing the original images and voila, I can now edit in PS. Thanks for the help guys, it was much appreciated!