Já tentou alterar o repositório para o 'servidor principal'? Sempre que você alterá-lo e fechar, ele solicitará que você recarregue. Isso deve restaurar os padrões.
Se isso não funcionar, aqui está uma cópia da minha:
deb http://archive.ubuntu.com/ubuntu xenial main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial restricted universe main multiverse
deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial-updates restricted universe main multiverse
deb http://archive.ubuntu.com/ubuntu xenial universe
deb http://archive.ubuntu.com/ubuntu xenial-updates universe
deb http://archive.ubuntu.com/ubuntu xenial multiverse
deb http://archive.ubuntu.com/ubuntu xenial-updates multiverse
deb http://archive.ubuntu.com/ubuntu xenial-security main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial-security restricted universe main multiverse
deb http://archive.ubuntu.com/ubuntu xenial-security universe
deb http://archive.ubuntu.com/ubuntu xenial-security multiverse
deb http://archive.canonical.com/ubuntu xenial partner
deb-src http://archive.canonical.com/ubuntu xenial partner