Why might apt-get not work on Ubuntu?

Question

linux

Hello colleagues.
Such a situation:
There is a machine with ubuntu 14.10 on board. For some reason, apt-get does not work, namely, when I do sudo apt-get mc, it writes E: Unable to locate package mc and so on with any packages.
The Internet was connected through a proxy with IP authorization. Configured a proxy: 1. Made export http_proxy, etc., 2. added proxy lines to files in /etc/apt/apt.conf and /etc/apt/apt.conf.d/proxy, in /etc/bash .bashrc, in /etc/environment. The result - the Internet works, wget downloads, apt-get refuses.
I spat and connected and configured the USB modem and cleaned the traces of the proxy on the computer - the essence is the same. The Internet works in the browser, YouTube shows, wget downloads, apt-get - No.
When executing apt-get update, in any case, it writes Err <address> Sources 404 Not Found
Where to dig, comrades?

Answer 1

Where to dig, comrades?

You don't have up-to-date lists of repositories in /etc/apt/sources.list The
best way to generate content is from here: Ubuntu Sources List Generator

Answer 2

and actually sources are described? check apt configs

Answer 3

Double check if the sources in /etc/apt/sources.list are correct
Temporarily disable third party sources (PPA) to be safe.

sudo apt-get update
sudo apt-get install mc

Answer 4

Thank you. Indeed, there were irrelevant links to the repositories.