I just heard that magical noise - ice cubes dropping! The line was indeed frozen. I was able to use a metal BBQ skewer to grind away at the plug until I saw dripping water. Nothing happened for a few hours and I wandered off to put the kids to bed and a moment ago I heard the noise I hadn't heard in a while...

Thank you! This forum, the how-to's and the parts service you all provide are invaluable!