Check your MSSQL connection configuration. Make a new file (called test.php or something) and put this inside of it:
Code:
<?php
$server = 'SOMETHING-PC\SQLEXPRESS'; // the name of your instance ( check SSMS if you don't know: http://i.imgur.com/2M77UMZ.png )
$user = 'sa'; // always 'sa'
$password = 'password'; //password you made when configuring SQL
// Connect to MSSQL
$link = mssql_connect($server, $user, $password);
if (!$link) {
die('Something went wrong while connecting to MSSQL');
}
else { echo 'MSSQL Connection Success'; }
?>
If this connects then your configuration for the website is wrong and needs to be changed, if it doesn't then you have some other error