I have had a rather annoying problem on my Ubuntu box for the last few days. Not sure how it happened, but my application menu suddenly stopped working. Whenever I clicked on the menu I just got a tiny grey box with nothing in it where the menu should have been.
After a lot of googling with no success, I finally stumbled on a post that mentioned some files in ~/.config/. When I had a look in there, there was a subdirectory called menus which contained a file called applications.menu. I had a look at that file, and sure enough it was empty. After having a quick look at my other Ubuntu box I noticed that box had no applications.menu, so I deleted the empty file and hey presto!