Primeiro limpar o cache do apt
sudo apt-get autoclean
Backup de todos os arquivos da lista de fontes.
sudo cp -a /etc/apt/sources.list /etc/apt/sources.list_backup
sudo cp -a /etc/apt/sources.list.d/ /etc/apt/sources.list.d_backup
Limpe o sources.list
com falha e remova seus arquivos.
sudo rm -f /etc/apt/sources.list
sudo rm -rf /etc/apt/sources.list.d/*
Criar novo sources.list
sudo nano /etc/apt/sources.list
Copie o conteúdo a seguir e salve o arquivo sources.list
#------------------------------------------------------------------------------#
# OFFICIAL UBUNTU REPOS #
#------------------------------------------------------------------------------#
###### Ubuntu Main Repos
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
###### Ubuntu Update Repos
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
Agora atualize os pacotes
sudo apt-get update && sudo apt-get upgrade