Multisite won't work on my website; I keep getting "error establishing database connection

I've tried twice to implement Multisite on my website. Every time I edit my wp-config and htaccess files with the code given on the network setup page, I receive an "Error establishing database connection" and I can't even log in to my site. I've heard WP has trouble converting existing sites to multisite if they're over a month old, but is there some workaround for that problem?