What is wrong with this code? I keep getting page not found

The address is the same, so why is it not finding the page?

<!-- Singin box section starts here -->
        <?php

			//If the signin button is pressed
			if (isset($_POST['action2']) && $_POST['action2'] == "signin") {

				$username = $_POST['username'];
				$password = $_POST['password'];
				$query = mysql_query("SELECT * FROM wp_users WHERE user_nicename='".$username."' and user_login='".$password."' ") or die(mysql_error());  

				//Set cookie ----------------------Starts here---------->
				if (mysql_num_rows($query)>0) {
					$user = $_POST['username'];
					setcookie("logon", $user, time()+3600*24 ,"/");
					$_COOKIE["user"] = $user;
				}

			}//endif signin

			//Check to see whether the cookie is set ( i.e. password is correct)
			if (isset($_COOKIE["user"])) { ?>
				<form id="commentform" name="commentform" method="post" action="">
					<p><label>Name: </label><input type="text" name="name" /></p>
					<p><label>E-mail:</label><input type="text" name="email" /></p>
					<p><label>Comment:</label><textarea name="comment" rows="7" style="width:250px" > </textarea></p>
					<p><button type="submit" name="action" value="comment_post" >Submit Comment</button></p>

				</form>
			<?php
				if (isset($_POST['action']) && $_POST['action'] == "comment_post") {
					echo 'Block exectuted';
					//insert the comment
					//mysql_query("INSERT INTO wp_comments (comment_author,comment_email,comment_date, comment_content) VALUES('".$_POST['name']."','".$_POST['email']."',NOW(),'".$_POST['comment']."','".$_POST['comment']."')") or die(mysql_error());
				}//endif comment posted

			} else {
				echo '<p>Password incorrect</p>';
				commentForm();
			}//endif password correct/incorrect
		?>
        </div>
        </div>
        <!-- Leave comment ends box -->
		<!-- Signin box section ends here -->