Initial troubleshooting steps:
1. Set Preferences > Performance > Use Graphics Processor to off. Does the problem go away?
2. Update your graphics driver by going directly to the manufacturer's web site:
3. Try resetting LR's preferences:
LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.
4. If the previous steps don't help, do the LR menu command Help > System Info and copy/paste the entire contents here so we can see exactly which versions of hardware and software LR thinks you're running.