uninstall everything cc including the cc desktop app.
clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html
restart your computer (don't skip this)
reinstall the cc desktop app, https://creative.adobe.com/products/creative-cloud
use the desktop app to install your cc programs/trials
if you've done all that (including the computer restart after cleaning), using a browser that allows popups and cookies, contact adobe support during pst business hours by clicking here and, when available, click 'still need help', http://helpx.adobe.com/x-productkb/global/service-ccm.html
I also just had Creative Cloud suddenly show up on yesterday's startup that the app needed to be reinstalled.
A nightmare has ensued that still isn't resolved. The uninstaller that comes with the app doesn't work. Needed to download the uninstaller online.
That worked, but still needed to do the third step of using the cleaner. Managed today to get the CC app to work, but now when I go to any of my apps, I once again get the nag screen that I need to install the desktop app, which conveniently cannot be installed when any of the apps are installed.
Thank you Adobe, for your software rental scheme that now prevents us from using our paid-for software.
No idea how to proceed now except to uninstall my entire suite and try from scratch.