我刚刚在计算机上安装了Lubuntu 19.04，一切正常。但是之后三天，apt update命令停止工作。我不知道是什么原因造成的。
这是我运行apt update的输出：
Ign:1 http://archive.ubuntu.com/ubuntu disco InRelease
Ign:2 http://security.ubuntu.com/ubuntu disco-security InRelease
Ign:3 http://archive.ubuntu.com/ubuntu disco-updates InRelease
Err:4 http://security.ubuntu.com/ubuntu disco-security Release
404 Not Found [IP: 91.189.88.152 80]
Ign:5 http://archive.ubuntu.com/ubuntu disco-backports InRelease
Err:6 http://archive.ubuntu.com/ubuntu disco Release
404 Not Found [IP: 91.189.88.142 80]
Err:7 http://archive.ubuntu.com/ubuntu disco-updates Release
404 Not Found [IP: 91.189.88.142 80]
Err:8 http://archive.ubuntu.com/ubuntu disco-backports Release
404 Not Found [IP: 91.189.88.142 80]
Reading package lists... Done
E: The repository 'http://security.ubuntu.com/ubuntu disco-security Release' does not have a Release file.
N: Updating from such a repository can't be done securely,and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco Release' does not have a Release file.
N: Updating from such a repository can't be done securely,and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely,and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://archive.ubuntu.com/ubuntu disco-backports Release' does not have a Release file.
N: Updating from such a repository can't be done securely,and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
这是我的sources.list文件。
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu/ disco main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ disco main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu/ disco-updates main restricted
# deb-src http://archive.ubuntu.com/ubuntu/ disco-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also,please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ disco universe
# deb-src http://archive.ubuntu.com/ubuntu/ disco universe
deb http://archive.ubuntu.com/ubuntu/ disco-updates universe
# deb-src http://archive.ubuntu.com/ubuntu/ disco-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team,and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also,please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu/ disco multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ disco multiverse
deb http://archive.ubuntu.com/ubuntu/ disco-updates multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ disco-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release,although it includes
## newer versions of some applications which may provide useful features.
## Also,please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu/ disco-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ disco-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu,but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu disco partner
# deb-src http://archive.canonical.com/ubuntu disco partner
deb http://security.ubuntu.com/ubuntu/ disco-security main restricted
# deb-src http://security.ubuntu.com/ubuntu/ disco-security main restricted
deb http://security.ubuntu.com/ubuntu/ disco-security universe
# deb-src http://security.ubuntu.com/ubuntu/ disco-security universe
deb http://security.ubuntu.com/ubuntu/ disco-security multiverse
# deb-src http://security.ubuntu.com/ubuntu/ disco-security multiverse
谁能说我该如何解决？ 显然，我无法安装任何东西：
apt install apache2
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package apache2
谢谢。