Today, I was experimenting with my Ubuntu, when I noticed that my Applications menu in my panel had no contents, only Places and System. I did a little search and found a solution in the Ubuntu forums. To restore it back, issue the following commands in the terminal.
rm /home/<your username>/.config/menus/applications.menu
This will remove your current configuration, and reset a new config. After that, your Applications menu should be restored back to default.