Won't connect to the database

Had a blog running and encountered a major problem and couldn’t display the site…..downloaded a fresh version of WP 3.0 replaced the ‘includes’ file…fixed that error.

Had to re-edit the config.php.

Used the db name from my cpanel, my SQL databases… I only have 1 db
Entered username and pwd.
If I leave the db location at localhost, I get nothing.

How do I specifiy the db location???

My WP files are installed in ‘mydomain.com/blog’
I’ve tried different variations and only get the “Error connecting to the database’ screen, or the WP screen advising it couldn’t connect and suggesting that I check the username,pwd and db location.

I figure the only thing would be the format of the db location.

Help please!

Thank you