I tried downloading and installing it again (free firewall version), incase that would provide a fix, but it won't install again, possibly because it's already installed.
Another thing that may or may not be related is that I noticed ZoneAlarm seemed to have forgotten all my earlier 'Accept' and 'Deny' decisions, as it suddenly started asking me, all over again, about things I've decided about more than a year ago.
But I mainly need the UI to show up for now, because I need to permanently allow an online game. Right now I have to enable 'Auto-accept' in "Game Mode" before I start the game everytime, or I'm looking at a frozen computer in need of restarting. However, these 'Auto-accepts' aren't permanent, so I need to get into the UI, which is now blank.