When some aspect of the GUI ceases to function, the first thing that I recommend is to go to the video card's mfgr's.Web site and download and install the latest video driver. Do not trust Windows, or any utility, to tell if you have the latest - check the number/date of the latest from nVidia, or ATI/AMD, vs what is installed.
If the updating of the video card's driver does not help things, see this ARTICLE. It starts with a checklist to get PrE working smoothly, and then goes into some tuneup tips for the system and the OS. Last, it covers a lot of troubleshooting tips.
If things have not been cleared up, then you will want to post the full details of your system, your Source Files and your Project's Preset. This ARTICLE goes into more detail.