root@s1 Downloads]# service mysql restart
MySQL server PID file could not be found!                  [FAILED]

[root@s1 Downloads]# service mysql stop
MySQL server PID file could not be found!                  [FAILED]

解决方法：
1.通过ss -tnl命令查看服务
[root@s1 Downloads]# ss -tnl
State       Recv-Q Send-Q   Local Address:Port   Peer Address:Port
LISTEN      0      50               *:3306                 *:* 
2，停止mysql服务
[root@s1 Downloads]# killall mysqld

3.重启mysql服务
4.发现mysql数据目录权限异常
[root@s1 Downloads]# mysql
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
5.修改mysql数据目录权限
[root@s1 Downloads]# chown -R mysql.mysql /var/lib/mysql/

6.重启服务，即可以正常登陆mysql