Lightroom 6.4 on my Windows 10 laptop suddenly stopped showing developed images in the develop modulel. Thumbnails are still present. When using split screen before-and-after, before is normal and after shows a solid blue panel. This is true of all images, old or new. I have reset all settings to default with no effect.
Uncheck the option Preferences > Performance > Use Graphics Processor.
Yup, that fixed it! Many thanks. It is an Intel "520" processor. I wonder whether it is Intel's or Microsoft's fault.