Go to Edit > Preferences > Performance, and und uncheck Use Graphics Processor.
If LR freezes/chrashes when you try to do this, try to go the web module first (before going to Edit < Prefs) , otherwise see this thread:
If this helps, try updating your graphics driver, and try turning it back on.
LR CC appears to have reverted to trial mode.
First log into adobe.com, click on Manage Account, and check to see that your Adobe ID is still associated with a CC plan that is in effect and hasn’t expired—after a year, or because of a CC payment error.
If the plan is current on your Adobe account, then here is a set of things to try: