Error establishing a database connection

Hi, I am fairly new to wordpress and I am enjoying it very much. I built my first webstore http://iphone-lady.info and it was up and running until yesterday.
I am getting an error message saying: error establishing a database connection. What may have happened: I didn’t have a password set in mysql but I included a DbUsername and password in wp-config.php. Is there a way to fix this problem without taking the site down and rebuilding it?

Gerry