In Ubuntu 12.04 after switching from Unity desktop to gnome classic Alt + F2 wont work by default.
To Enable it you need to change following settings
Go to Settings –> KeyBoard –> Shortcuts –> System. Now click on “Show the run command prompt”. This will ask you for new accelerator, press Alt + F2 here and close the window.
Now thy pressing Alt + F2 it will show you run command prompt.