This is a generic message which usually indicates your site is having problems accessing data in your MySQL database.
It could be a problem with:
- MySQL not running
- incorrect DB_xxxxxxx settings in either of your configure.php files
- syntax error in MySQL statements
- an incompatible addon, on an incorrectly-installed addon (such as SQL installation steps not completed properly)
- a hacker attempting to do something rogue or malicious with something