Did you have GCA installed before, and do you have multiple monitors? Because this error pops up a lot when you have multiple monitors and the Edit or other sub-windows are actually opening up off screen, usually in a position where a monitor used to be positioned. If you can press Escape and GCA starts responding again this is almost certainly what's happening.
The surest way to fix this is to reset GCA's personalized settings by running GCAClearAppData as
outlined in the FAQ.
You can see another, slightly more detailed, response to this issue in this thread:
http://forums.sjgames.com/showthread.php?t=150801