由于不小心删除了 /var/log/mysql/ 这个目录（这是mysql服务器用来保存log的地方），导致无法启动mysqld。

在终端输入 service mysql start 回车，输出：

Job for mysql.service failed because the control process exited with error code.
See "systemctl status mysql.service" and "journalctl -xe" for details.

于是在终端输入 mysqld 回车，输出：

2020-12-24T06:29:14.144840Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2020-12-24T06:29:14.146597Z 0 [Note] mysqld (mysqld 5.7.31-0ubuntu0.18.04.1) starting as process 2760 ...
2020-12-24T06:29:14.151841Z 0 [ERROR] Could not open file '/var/log/mysql/error.log' for error logging: Permission denied
2020-12-24T06:29:14.151990Z 0 [ERROR] Aborting

2020-12-24T06:29:14.152063Z 0 [Note] Binlog end
2020-12-24T06:29:14.152108Z 0 [Note] mysqld: Shutdown complete

从提示看，是无法创建error.log，于是创建/var/log/mysql，再执行 ls -l /var/log 发现mysql这个目录的用户权限和组权限都是root，这就是问题所在，需要将其都改为mysql（使用chmod和chown命令）。