I have LR 5.7.
Some weeks ago it stopped creating previews upon import It will not let me build them either. All I get is grey boxes in the Library middle. In Develop it will only show 3 previews in any one folder at a time
I have tried re-installing, deleting cache. Deleting lr.cat preview file and many other things.
Any advice would be welcome.
What do you mean with "In Develop it will only show 3 previews ..."? Develop shows only one photo. Do you see a photo in develop to work on?
What is ypour OS?
If you see only gray boxes, your moitor profile could be corrupt. For the purpose of checking this, select the default sRGB System profile and see if this helps.