Connection timed out and lock_may_be_available() on working site
There could be multiple reasons/thoughts: 


Are you sure mysql is running? 
Has your site got just too big for the resources allocated to mysql in the configuration?
Did you manually "emptied" all of the cache files via the database?
Low file system space on the database server
If you just moved the site to the other server and mysql is not set up to accept enough connections. 
The user you are connecting with has SELECT permissions on mysql but not DELETE or UPDATE etc.
The host settings in sites/default/settings.php doesn't matche the one from the User in the database
MySQL 5.7 does not allow root user to access a database from client app. After switching to a normal user, it solves.
Sometimes tuning xampp like on php.ini (memory_limit=256M) and my.ini (max_allowed_packet = 32M) and disable devel by drush (drush dis -y devel) also works.
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