Starting MySQL.. ERROR! The server quit without updating PID file (/var/lib/mysql/domainname.com.pid).

In this case we need to do lot of troubleshooting methods

#mv /etc/my.cnf /etc/my.cnf.bak
#service mysqld restart

check either mysql process running background just kill and restart

#ps -aux |grep mysql

#kill -9 task id

very important folder have to check

#cd /var/lib/mysql
#mv /var/lib/ib_logfile* /var/root/backup

#service mysqld restart

check the error messages

#vi /var/lib/mysql/hostname.err

# chown -R mysql /var/lib/mysql

check mysql.sock if not found then

#find / - name mysql.sock
//based on the result please link to /var/lib/mysql/mysql.sock

If you seen any plugins issue then
# cd /var/lib/mysql/mysql/
# wget https://support.plesk.com/hc/en-us/article_attachments/115000459433/plugin.tar
# tar zxvf plugin.tar
# chown mysql.mysql /var/lib/mysql/mysql/plugin*
# service mysql restart

If none of the above is working, Finally we can do for safe start

#mysqld_safe --skip-grant-tables

//if safe star run your front end will be work properly by you not able to access your phpmyadmin page
//if it's run can can do for repair the mysql

#service mysql start --skip-grant-tables

// now mysql will start then

#mysqlcheck -r mysql  //it will get some time to resolve

#mysqlcheck -r mysql db  //this also doing debug

after finished the debug

#service mysql stop

#service mysql start
Success!