1.замените содержимое файла - /etc/apt/sources.list
на
deb http:// archive.canonical.com/ubuntu trusty-proposed partner
deb http:// archive.canonical.com/ubuntu trusty partner
deb http:// extras.ubuntu.com/ubuntu trusty main
deb http:// security.ubuntu.com/ubuntu trusty-backports universe main multiverse restricted
deb http:// security.ubuntu.com/ubuntu trusty-proposed universe main multiverse restricted
deb http:// security.ubuntu.com/ubuntu trusty-security universe main multiverse restricted
deb http:// security.ubuntu.com/ubuntu trusty-updates universe main multiverse restricted
deb http:// security.ubuntu.com/ubuntu trusty universe main multiverse restricted
deb http:// archive.ubuntu.com/ubuntu trusty-backports universe main multiverse restricted
deb http:// archive.ubuntu.com/ubuntu trusty-proposed universe main multiverse restricted
deb http:// archive.ubuntu.com/ubuntu trusty-security universe main multiverse restricted
deb http:// archive.ubuntu.com/ubuntu trusty-updates universe main multiverse restricted
deb http:// archive.ubuntu.com/ubuntu trusty universe main multiverse restricted
2. дальше сделайте обновление системы в терминале
$sudo apt-get update
$sudo apt-get dist-upgrade
3. перезагрузитесь.
о результатах отпишитесь здесь.
если не поможет, будем думать дальше.