Terminal: sudo: apt-get: command not found

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http://askubuntu.com – This is my first post here, so let me be welcomed with a serious issue to be answered. Lately some really bad things have been happening with my computer, having to do with lack of icons and all kinds of bad things going with my graphical interface. All the bad things in the appeal are accompanied by a certain disability of my terminal. I'm using 12.04 with Gnome Classic view with no effects. Let's have a "sudo apt-get update". I get "sudo: apt-get: command not found" message. And it is the way it goes with all my apt-get commands. So, guys - what can it mean? How to fix this, especially co (HowTos)