Try resetting Lightroom preferences:
Hint: on my Windows 7 machine it's difficult (impossible?) to hold down Alt+Shift and double-click on the Lr desktop shortcut; instead, hold down Alt then hold down Shift then right-click the Lr shortcut and choose Open.
If you reset your preferences without noting your current catalog name and location, it can appear that your catalog has disappeared when you restart Lightroom. Before you reset your preferences, refer to this technote, which contains steps to avoid this confusion.
Many thanks for your suggestion,
I tried it but it did not solve the problem.
I have tried resetting preference, exiting adobe CC and re-signing in numerous times
There appears from other users questions to be a bug in the latest version of Lightroom 6 that is not fixed by the latest update. Lightroom 5 works fine on my PC as does Photoshop CC from which I conclude the problem is with Lightroom 6
Any further suggestions gratefully received