I lost the system menu

I am new to Ubuntu, so as I am playing around and making changes here and there, I lost the system menu on the left hand menu bar. And of course, now, I can not access any of the system applications. I have read a couple of questions and answers relating to this same kind of problem and have tried with no success. I am unable to use the alt-f2 key combination because my keyboard will change the screen brightness. If I try to right click on any empty place of the menu bar system, instead of getting access to the menu items, all I get is the app to change the desktop appearance. So as you can see it is a catch-22.

Thank you guys, in advance, for any help rendered and please go easy with whatever response you may give.

Carlos