Please post a screen capture.
What is your exact version of LR and your OS?
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A couple of possibilities come to mind-
1. The Preview cache has become corrupted.
2. Windows possibly changed the Graphics drivers or the Monitor Profile.
Try changing the monitor profile- Photos appear as black, gray, or blank rectangles in Lightroom
Read the section: Solution 2: Associate sRGB IEC61966-2.1 Color profile on your system
and check you have the best drivers for your Graphics card.
Thank you so much. have tried no 2 but didn't work so now going to try no 1!!!