Try logging out of the CC app, then back in again. Then try LR CC again.
It seems to have worked ! I was doubtful since I had already rebooted so I thought that would resolve things.
After the logout/login, I launched LR and updated my default catalogue. Then notified me that a third-party plugin was no longer compatible (no problem, I dont use it anyway)
Now it is working
I tried that, it does nothing. I uninstalled the new update and reinstalled, then restarted the computer (Mac Yosemite) and now I can't even run the old version. When I click on the new app (either within the CC window or using Finder) the new LR screen image flashes for a fraction of the second, then nothing happens. Adobe has made a colossal mess of this introduction of LR CC.
Logging out and back in worked for me. Thanks!