PC locks up when using the ellipsis to choose a variable or FB in Unity Pro.



This usually occurs if multiple monitors had been used and one or more of them is now gone.
If the last time Unity was used the pop up window was on a monitor that is no longer there you will not see it. 

After lock up select 'ALT & Space' together, release those keys and select 'M' .This will allow you to move the lost window into view using the keyboard arrows.