Menu Bars Frozen / Panel Not Responding

I have Ubuntu -- the version just before the very latest one -- and I am using a desktop system (my system is only part-functional at this moment, so I can't access the details). I was moving menu-bars on the opening screen, on the desktop, by right-clicking on them. I moved the Applications/Places/System menu bar to the bottom of the screen, making this a fixed menu bar. Then I moved the active applications menu bar to the bottom of the screen also, making it a popup menu bar. At this point, the Applications/Places/System menu bar froze (and is still frozen), and the active applications menu bar vanished, and cannot be called up. Rebooting the computer does not solve the problem. So I have essentially lost all access to my applications. However, I can trick the system into displaying Nautilus, and I can call up a few applications through the associated files. I can also access the command prompt at boot-up, although I do not know how to use it. I followed some advice on Launchpad to press Alt-F2. However, there was no response to this. When shutting down my system (by pushing the power button, since the menu bar is frozen), the computer reports "Panel not responding". I would be most grateful for any advice that could get me out of this jam.