Problem:

• All Joomla websites hosted on single virtual server are down.
• Trying to access any of the sites, web browsers display following errors:
  Infinite loop detected in JError
  or
  Database Error: Unable to connect to the database:Could not connect to MySQL
• phpMyAdmin web interface displays login screen would not allow to login (simply reloads the page after submitting credentials).
• Trying to connect to MySQL via console fails with errors:
  ERROR 1040 (hy000): Too many connections
  or
  ERROR 2002   (HY000): Can’t connect to local MySQL server through socket ‘/var/run/mysqld/mysqld.sock’ (2)

Cause

mySQL log file /var/log/mysql/error.log finally revealed that the problem was caused by lack of space on the disk:
[ERROR] /usr/sbin/mysqld: Disk is full writing ‘./<database_name>/<table_name>.MYD’ (Errcode: 28). Waiting for someone to free space… (Expect up to 60 secs delay for server to continue after freeing disk space)

Solution:

Pretty obvious – free up some space on the hard drive.