What does this error mean?

Question

[Thu Feb 11 02:03:48 2016] [error] [client 54.94.140.139] PHP Warning:  mysqli::mysqli(): (HY000/2002): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) in /var/www/vhosts/spaceamigos.com/httpdocs/qa-include/qa-db.php on line 66

Comments

It means PHP can't connect to your database. Either the login details are wrong, or MySQL isn't running. Is this occurring on a specific page?

Maybe this will help: http://stackoverflow.com/questions/13769504/mysqlimysqli-hy000-2002-cant-connect-to-local-mysql-server-through-sock

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The whole site was unavailable.
I rebooted the server and then the site back up and running.
However, in busy times it becomes unavailable again.
Thanks.