restart of lightdm gives blank screen

[Linda]

I have a computer I installed Lubuntu on several weeks ago. 
It was working fine to log off as one user and back on as 
another. This weekend it started to give a blank screen if 
it goes to screen lock or log out. I can ssh in and reboot 
then you can login, however if I ssh in before anyone has 
loged on and restart lighdm you also get a blank screen I 
get the notice that lightdm has restarted (lightdm 
start/running, process 1948) but have a blank screen.  Any 
idea what is causing this and how I can fix it.
          Thanks
           Linda