E: Some index files failed to download. They have been ignored, or old ones used instead after apt-get update

My /etc/apt/sources.list contained:

nazar_art@nazar-desctop:/etc/apt$ cat source.list
deb http://security.ubuntu.com/ubuntu precise-security main universe
nazar_art@nazar-desctop:/etc/apt$ cat sources.list.old
deb http://archive.ubuntu.com/ubuntu precise universe
nazar_art@nazar-desctop:/etc/apt$ cat source.list
deb http://security.ubuntu.com/ubuntu precise-security main universe
nazar_art@nazar-desctop:/etc/apt$ 

I updated according to this post How do I restore the default repositories?

But It called another problem when I run sudo apt-get update I have some output like this:

Err http://ua.archive.ubuntu.com main/restricted Sources                       
  404  Not Found [IP: 91.189.91.13 80]
Err http://ua.archive.ubuntu.com main/universe Sources                         
  404  Not Found [IP: 91.189.91.13 80]
Err http://ua.archive.ubuntu.com main/multiverse Sources                       
  404  Not Found [IP: 91.189.91.13 80]
Err http://ua.archive.ubuntu.com main/restricted amd64 Packages                
  404  Not Found [IP: 91.189.91.13 80]
Err http://ua.archive.ubuntu.com main/universe amd64 Packages                  
  404  Not Found [IP: 91.189.91.13 80]

and this:

W: Failed to fetch http://ua.archive.ubuntu.com/ubuntu/dists/-backports/restricted/binary-i386/Packages  404  Not Found [IP: 91.189.91.13 80]

W: Failed to fetch http://ua.archive.ubuntu.com/ubuntu/dists/-backports/universe/binary-i386/Packages  404  Not Found [IP: 91.189.91.13 80]

W: Failed to fetch http://ua.archive.ubuntu.com/ubuntu/dists/-backports/multiverse/binary-i386/Packages  404  Not Found [IP: 91.189.91.13 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

UPDATE:

I did this command (Thanks NikTh):

find /etc/apt -name '*.list' -exec bash -c 'echo -e "\n$1\n"; cat -n "$1"' _ '{}' \;

whole output:

/etc/apt/sources.list

     1  #############################################################
     2  ################### OFFICIAL UBUNTU REPOS ###################
     3  #############################################################
     4  
     5  ###### Ubuntu Main Repos
     6  deb http://ua.archive.ubuntu.com/ubuntu/  main restricted universe multiverse 
     7  deb-src http://ua.archive.ubuntu.com/ubuntu/  main restricted universe multiverse 
     8  
     9  ###### Ubuntu Update Repos
    10  deb http://ua.archive.ubuntu.com/ubuntu/ -security main restricted universe multiverse 
    11  deb http://ua.archive.ubuntu.com/ubuntu/ -updates main restricted universe multiverse 
    12  deb http://ua.archive.ubuntu.com/ubuntu/ -proposed main restricted universe multiverse 
    13  deb http://ua.archive.ubuntu.com/ubuntu/ -backports main restricted universe multiverse 
    14  deb-src http://ua.archive.ubuntu.com/ubuntu/ -security main restricted universe multiverse 
    15  deb-src http://ua.archive.ubuntu.com/ubuntu/ -updates main restricted universe multiverse 
    16  deb-src http://ua.archive.ubuntu.com/ubuntu/ -proposed main restricted universe multiverse 
    17  deb-src http://ua.archive.ubuntu.com/ubuntu/ -backports main restricted universe multiverse 
    18  
    19  
    20  
    21  #############################################################
    22  ################### OFFICIAL UBUNTU REPOS ###################
    23  #############################################################
    24  
    25  ###### Ubuntu Main Repos
    26  deb http://ua.archive.ubuntu.com/ubuntu/  main restricted universe multiverse 
    27  deb-src http://ua.archive.ubuntu.com/ubuntu/  main restricted universe multiverse 
    28  
    29  ###### Ubuntu Update Repos
    30  deb http://ua.archive.ubuntu.com/ubuntu/ -security main restricted universe multiverse 
    31  deb http://ua.archive.ubuntu.com/ubuntu/ -updates main restricted universe multiverse 
    32  deb http://ua.archive.ubuntu.com/ubuntu/ -proposed main restricted universe multiverse 
    33  deb http://ua.archive.ubuntu.com/ubuntu/ -backports main restricted universe multiverse 
    34  deb-src http://ua.archive.ubuntu.com/ubuntu/ -security main restricted universe multiverse 
    35  deb-src http://ua.archive.ubuntu.com/ubuntu/ -updates main restricted universe multiverse 
    36  deb-src http://ua.archive.ubuntu.com/ubuntu/ -proposed main restricted universe multiverse 
    37  deb-src http://ua.archive.ubuntu.com/ubuntu/ -backports main restricted universe multiverse 
    38  
    39  

/etc/apt/source.list

     1  deb http://security.ubuntu.com/ubuntu precise-security main universe

/etc/apt/sources.list.d/ubuntu-extras.list

     1  deb http://extras.ubuntu.com/ubuntu/ precise main #Added by software-center
     2  deb-src http://extras.ubuntu.com/ubuntu/ precise main #Added by software-center

/etc/apt/sources.list.d/webupd8team-y-ppa-manager-precise.list

     1  deb http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu precise main

/etc/apt/sources.list.d/playonlinux.list

     1  deb http://deb.playonlinux.com/ quantal main

/etc/apt/sources.list.d/private-ppa.launchpad.net_commercial-ppa-uploaders_intellij-idea-ce_ubuntu.list

     1  deb https://private-ppa.launchpad.net/commercial-ppa-uploaders/intellij-idea-ce/ubuntu precise main #Added by software-center; credentials stored in /etc/apt/auth.conf

/etc/apt/sources.list.d/webupd8team-sublime-text-3-precise.list

     1  deb http://ppa.launchpad.net/webupd8team/sublime-text-3/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/webupd8team/sublime-text-3/ubuntu precise main

/etc/apt/sources.list.d/medibuntu.list

     1  ## Please report any bug on https://bugs.launchpad.net/medibuntu/
     2  deb http://packages.medibuntu.org/ precise free non-free #Medibuntu - Ubuntu 12.04 "precise pangolin"
     3  # deb-src http://packages.medibuntu.org/ precise free non-free #Medibuntu (source) - Ubuntu 12.04 "precise pangolin"

/etc/apt/sources.list.d/lezhoev-qstamina-precise.list

     1  deb http://ppa.launchpad.net/lezhoev/qstamina/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/lezhoev/qstamina/ubuntu precise main

/etc/apt/sources.list.d/webupd8team-java-precise.list

     1  deb http://ppa.launchpad.net/webupd8team/java/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu precise main

/etc/apt/sources.list.d/tualatrix-ppa-precise.list

     1  deb http://ppa.launchpad.net/tualatrix/ppa/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/tualatrix/ppa/ubuntu precise main

/etc/apt/sources.list.d/kilian-f_lux-precise.list

     1  deb http://ppa.launchpad.net/kilian/f.lux/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/kilian/f.lux/ubuntu precise main

/etc/apt/sources.list.d/ubuntu-wine-ppa-precise.list

     1  deb http://ppa.launchpad.net/ubuntu-wine/ppa/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/ubuntu-wine/ppa/ubuntu precise main

/etc/apt/sources.list.d/fsqcds-goldendict-precise.list

     1  deb http://ppa.launchpad.net/fsqcds/goldendict/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/fsqcds/goldendict/ubuntu precise main

/etc/apt/sources.list.d/webupd8team-rhythmbox-precise.list

     1  deb http://ppa.launchpad.net/webupd8team/rhythmbox/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/webupd8team/rhythmbox/ubuntu precise main

/etc/apt/sources.list.d/upubuntu-com-devel-precise.list

     1  deb http://ppa.launchpad.net/upubuntu-com/devel/ubuntu precise main
     2  deb-src http://ppa.launchpad.net/upubuntu-com/devel/ubuntu precise main

• How to solve this trouble in Ubuntu 12.04

Open a terminal (CTRL+ALT+T) and issue the following commands in order.

SOLUTION:

sudo cp /etc/apt/sources.list ~/ 
sudo wget "http://pastebin.com/raw.php?i=uzhrtg5M" -O /etc/apt/sources.list 
sudo apt-get update
sudo rm /etc/apt/sources.list.d/ubuntu-extras.list
sudo apt-get update

This solution is not general. It is specific to the sources.list file of the Original Poster.

The second command which starts with wget downloads and replaces the sources.list file with a specific for Precise (Ubuntu 12.04) sources.list file. See the file in raw format by visiting this page.

The fourth command removes the ubuntu-extras.list file which had an impact/conflict with another file of the same, so we removed it to avoid the error

W: Duplicate sources.list entry

see comments above.

It means that these sources cannot be reached:

http://ua.archive.ubuntu.com/ubuntu/dists/-backports/restricted/binary-i386/Packages  
http://ua.archive.ubuntu.com/ubuntu/dists/-backports/universe/binary-i386/Packages   
http://ua.archive.ubuntu.com/ubuntu/dists/-backports/multiverse/binary-i386/Packages  

Try selecting another server to fetch from (not the one in Ukraine).