Thanks for your help, Chuck. I deselected all javascript files in the minify area, intending to add them back in one by one, as you suggested. Then, I cleared the cache. But my navigation is still broken.
I also don't know whether to select "combine only" or "minify" for "head", "after body" and "before /body".
What with all the settings, and the various .js & .css files, there are hundreds of combinations...
Funny thing is it was working perfectly last week...now it looks just terrible.