WordPress network: "error establishing database connection"

After running WP 3.1.3 for just a few weeks on a new site, I decided to activate the “network” settings. I am using subdomains and am hosting at Hostgator. All seemed to go fine. I can still access my original site at http://99andunder.com. I can also use the network admin features to create a new site (seemingly) successfully.

However, the new site (such as sf2.99andunder.com) is inaccessible. I get the error in my browser “Error establishing a database connection.” I have read a few other related threads and tried disabling all plugins before creating a new site, but still the same error. Am not a database expert, so I have not mucked w/MySQL settings.

Any advice? I am happy to post whatever config or file information is needed. Thanks in advance.