When does it happen?
Is there any error message?
Chat computer and operating system are you using and how much memory do you have (RAM)?
Have you tried changing the GPU settigns?
Edit >> Preferences >> Performance
Lightroom >> Preferences >> Performance (on Mac OSX)
Then disable GPU acceleration.
For Windows you should go to the website of the manufacturer of your graphics processor and check for driver updates. After updating you could try switching GPU acceleration on again, otherwise leave it disabled.
On mac GPU updates happen with OS updates.
I've fixed it by downloading LR6 again and simply reinstalling. Thanks.