This usually indicates that the info in config.php is incorrect
Download that file with ftp and check that it does contain the correct database connection details.
This is what i currently have..
// phpBB 2.x auto-generated config file
// Do not change anything in this file!
$dbms = 'mysql4';
$dbhost = 'localhost';
$dbname = 'cornerst_p_bb2';
$dbuser = 'cornerst_bb2';
$dbpasswd = '*****';
$table_prefix = 'phpbb_';
my actual database name is cornerst_p_bb1 so all i should have to do is take off the 2 and replace with a 1 correct? i tried that but im still getting errors