putting www back in front of domain name

Hi all,

I installed wordpress today by uploading it through ftp. Everything worked fine, I had a domain name, hosting account, MySQL database set up etc and the
wp-config.php edited properly. I started building the website so everything worked. I had www in front of the domain name also.

But I messed up and decided to delete the wordpress files on the ftp site and upload them again.

But now the www part of the domain name is gone. So I re-uploaded the wordpress files through ftp to their original position.

But when I go to mysite.com/wp-admin/intall.php its says it cannot establish a database connection, i.e there is something wrong with the wp-config file. But I made no changes to the file or edited the My SQL database details.

The only thing different is that the domain name does not have www at the start any more.

I know wordpress automatically removes it, but is there anyway of getting www back in my domain name so I can try fix this?

All advice welcome

John