I made changes in wp-config.php before but I edit the changes back because it did not work right now. I tried to change the domain settings.

service mysql restart - not working. Only sudo command is working.

If I try to access /var/log/mysql/ then messages appears:
Error: Directory /var/log/mysql: permission denied
Error: Failed to retrieve directory listing

I was able to accès log files from FileZilla.
I used Terminal and command sudo cat /var/log/syslog to access the file.

Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24583]: Upgrading MySQL tables if necessary.
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24586]: /usr/bin/mysql_upgrade: the '--basedir' option is always ignored
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24586]: Looking for 'mysql' as: /usr/bin/mysql
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24586]: Looking for 'mysqlcheck' as: /usr/bin/mysqlcheck
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24586]: This installation of MySQL is already upgraded to 5.5.49, use --force if you still need to run mysql_upgrade
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24597]: Checking for insecure root accounts.
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24602]: WARNING: mysql.user contains 3 root accounts without password!
Jul  6 07:04:19 wordpress-1gb-ams3 /etc/mysql/debian-start[24603]: Triggering myisam-recover for all MyISAM tables
Jul  6 07:09:01 wordpress-1gb-ams3 CRON[24804]: (root) CMD (  [ -x /usr/lib/php5/sessionclean ] && /usr/lib/php5/sessionclean)
Jul  6 07:17:01 wordpress-1gb-ams3 CRON[24846]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jul  6 07:39:01 wordpress-1gb-ams3 CRON[25117]: (root) CMD (  [ -x /usr/lib/php5/sessionclean ] && /usr/lib/php5/sessionclean)
Jul  6 08:09:01 wordpress-1gb-ams3 CRON[25569]: (root) CMD (  [ -x /usr/lib/php5/sessionclean ] && /usr/lib/php5/sessionclean)
Jul  6 08:17:01 wordpress-1gb-ams3 CRON[25683]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)

I got mysql error also

sudo cat /var/log/mysql/error.log

160706  7:04:15 [Note] /usr/sbin/mysqld: Normal shutdown

160706  7:04:15 [Note] Event Scheduler: Purging the queue. 0 events
160706  7:04:15  InnoDB: Starting shutdown...
160706  7:04:17  InnoDB: Shutdown completed; log sequence number 59577283
160706  7:04:17 [Note] /usr/sbin/mysqld: Shutdown complete

160706  7:04:17 [Warning] Using unique option prefix myisam-recover instead of myisam-recover-options is deprecated and will be removed in a future release. Please use the full name instead.
160706  7:04:17 [Note] Plugin 'FEDERATED' is disabled.
160706  7:04:17 InnoDB: The InnoDB memory heap is disabled
160706  7:04:17 InnoDB: Mutexes and rw_locks use GCC atomic builtins
160706  7:04:17 InnoDB: Compressed tables use zlib 1.2.8
160706  7:04:17 InnoDB: Using Linux native AIO
160706  7:04:17 InnoDB: Initializing buffer pool, size = 128.0M
160706  7:04:17 InnoDB: Completed initialization of buffer pool
160706  7:04:17 InnoDB: highest supported file format is Barracuda.
160706  7:04:17  InnoDB: Waiting for the background threads to start
160706  7:04:18 InnoDB: 5.5.49 started; log sequence number 59577283
160706  7:04:18 [Note] Server hostname (bind-address): '127.0.0.1'; port: 3306
160706  7:04:18 [Note]   - '127.0.0.1' resolves to '127.0.0.1';
160706  7:04:18 [Note] Server socket created on IP: '127.0.0.1'.
160706  7:04:18 [Note] Event Scheduler: Loaded 0 events
160706  7:04:18 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.49-0ubuntu0.14.04.1'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  (Ubuntu)