n00854180t first find php.ini file and edit Error handling and logging:
error_reporting = E_ALL & ~E_NOTICE
to
error_reporting = E_ALL
i hope you have a test server where u can do that and you will get all errors in your code
Second:
$txtUser = $_POST['txtFName'];
i dont know where from u get variable $_POST['txtFName'], from form u have $_POST['txtUser']
Third:
$sql = "SELECT * FROM rcold_users WHERE uFirstName='$txtUser' AND uPass='$txtPass)';";
/* shuld be: */
$sql = "SELECT * FROM rcold_users WHERE uFirstName='$txtUser' AND uPass='$txtPass'";