The Linux Kernel offers you something that allows you to recover your system from a crash or at the least lets you to perform a proper shutdown using the Magic SysRq Keys. The magic SysRq key is a select key combination in the Linux kernel which allows the user to perform various low level commands regardless of the system’s state using the SysRq key.

Despite jeering at Windows for the infamous system freezes and blue screens of death, there are and will be times when your computer just locks up: the cursor is frozen and even invoking a console by Ctrl + Alt + [F2, F3, ...] to close down the X windows session running on F7 is non-functional.

The magic SysRq keys are key combinations within the Linux kernel that allows the user to perform various low level commands regardless of the system's state, except during kernel panics or freezes. It is often used to recover from X-Server freezes, or to reboot a computer without corrupting the filesystem.

Reseting Gnome Settings

If your gnome-panel corrupt , Application Menu, Places Menu and System Menu and some other menu items will disappear.
How to recover it

Boot the system upto Login Window. Press Ctrl + Alt+F1

Now You will get a Text based login screen

login with your username and password

now you will get a $ prompt.

"Hi, We just edited our hint files, so that all packages now need human review in order to go to testing. Hmm, that sounds too cryptic. Let's try again: Lenny is now frozen! Wheeeeeee!!! Thanks are due to everyone who has helped get us to this point [...] Now to explain what, exactly, we mean by 'freeze'..."

This tutorial will explain how to recover disappeared Application Menu, Places Menu and System Menu in ubuntu
Procedure to follow...

Just follow the below steps to recover it.

1. Reboot your system and at the GRUB press "e". Edit the first line and make sure it says "linux single". Then press "b" to boot the new entry line.

Since Ubuntu 9.04, the Ctrl-Alt-Backspace key combination to force a restart of X is now disabled by default, to eliminate the problem of accidentally triggering the key combination. In addition, the Ctrl-Alt-Backspace option is now configured as an X keymap (XKB) option, replacing the X server "DontZap" option and allowing per-user configuration of this setting.

The ctrl-alt-backspace key combination was used to restart X-server and was recently disabled because of a upstream (X.org) design decision: