MySql server PID not found

I have CentOS 6.4 with NGINX.

When I try to start/stop/restart mysql server (/etc/init.d/mysqld restart) I get this error:

MySQL server PID file could not be found!                  [FAILED]
Starting MySQL..The server quit without updating PID file ([FAILED]/mysql/mysqld.pid).

What can I do to solve this problem?

Thanks!