I cannot set up display references in OX 10.7.5. When I plug in external apple display both screens go blank.

I cannot set up display references in OX 10.7.5. When I plug in external apple display both screens go blank (to a bluegray colour). If I close the lid on pro the apple 27 inch display works. If I unplug the display the pro screen works. Problem is that I cannot set up preferences for displays. Until I downloaded an update I could set the display preferences ie. mirror or chose the main monitor etc. That is gone.

If I close the cover on the mac pro the apple display works. But in this case I cannot use the keyboard ont the mac pro because as soon as I lift the lid both screens go blank - to the dark blue grey colour.

Thank you for your assistance.