Re: Cannot select DB error!
My guess is that there is something wrong with the database settings in config.php (sorry) – if it is using the same database as the WordPress site, and the WordPress site is still up, then I think there’s something wrong with the config.php. Unless you changed something else recently with your files, I can’t think of anything else that would cause it.
Since posting an edited wp-config.php and config.php would be useless since you’d be destroying information, I would have another set of eyes look at both config files and see if there are any differences. Maybe the config.php was edited with some editor that mangled a character or line ending or something.
Also, can you connect to the database with phpMyAdmin or something using the details from config.php? Make sure the database is still there?