Generally, there are two possibile causes for this error.  MySQL is down or it has run out of database connections.  If it is the former, there is little you can do about it apart from contact your Host/server admin.

However, if the first is ruled out, then try going into your config.php file and changing

to

This will force ExpressionEngine to use non-persistent connection, which is far less likely to cause MySQL to run out of database connections.