One other thing i noticed that might be causing you a problem is your bind_param() might be throwing "ERROR: Could not prepare SQL statement.""
try changing :
$stmt->bind_param("ss", $clientname, $address, $city, $telephoneno, $mobileno, $email, $sno);
to
$stmt->bind_param("sssssss", $clientname, $address, $city, $telephoneno, $mobileno, $email, $sno);
as I believe you have to specify the type of each value (though i might be wrong, I dont use bind_param() very often, i hope someone corrects me on this if i am incorrect See more info @ "http://php.net/manual/en/mysqli-stmt.bind-param.php" )