我刚刚在计算机上安装了 Lubuntu 19.04，一切正常。但 3 天后，该apt update命令停止工作。我不知道是什么原因造成的。这是我运行的输出apt update：
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 …ubuntu-19.04 ×1