PHP Update password script doesnt work - php

Question

I'm working on a website with a register system. In the userCP you can change your password. I made this script for it but it doesnt work. Can somebody help me? When I change the password it doesnt give an error but it just doesnt update it.
PHP Code:
<?php
if (isset($_POST['updatePassBtn']))
{
$cpassword = $_POST['cpassword'];
$npassword = $_POST['npassword'];
$rpassword = $_POST['rpassword'];
if (!empty($cpassword) && !empty($npassword) && !empty($rpassword))
{
if ($npassword == $rpassword)
{
if (mysql_num_rows(mysql_query("SELECT * FROM `users` WHERE `username` = '".$_SESSION['username']."' AND `password` = '".SHA1($cpassword)."'")))
{
mysql_query("UPDATE `users` SET `password` = '".SHA1($npassword)."' WHERE `username` = '".$_SESSION['username']."' AND `ID` = '".$_SESSION['id']."'");
echo '<div class="nNote nSuccess hideit"><p><strong style="color:green;">SUCCESS: </strong>Password Has Been Updated</p></div>';
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong style="color:red;">FAILURE: </strong>Current Password is incorrect.</p></div>';
}
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong style="color:red;">FAILURE: </strong>New Passwords Did Not Match.</p></div>';
}
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong style="color:red;">FAILURE: </strong>Please fill in all fields</p></div>';
}
}
?>
My form:
<form action="" class="form" method="POST">
<fieldset>
<label></label>
<input name="cpassword" type="text" value="Current Password" onfocus="this.value = (this.value=='Current Password')? '' : this.value;" onblur="if(this.value == ''){this.value='Current Password';}"/>
</fieldset>
<fieldset>
<label></label>
<input name="npassword" type="text" value="New Password" onfocus="this.value = (this.value=='New Password')? '' : this.value;" onblur="if(this.value == ''){this.value='New Password';}"/>
</fieldset>
<fieldset>
<label></label>
<input name="rpassword" type="text" value="Repeat Password" onfocus="this.value = (this.value=='Repeat Password')? '' : this.value;" onblur="if(this.value == ''){this.value='Repeat Password';}"/>
<input type="submit" value="Update" name="updatePassBtn"/>
</fieldset>
</form>

Answer 1

You count the number of rows that match the username and password, but then when you update you also have the condition that it has to match $_SESSION['id']. If your session doesn't contain the right 'id', then your update could match no rows.
You should check mysql_affected_rows() before reporting that the update was successful.
You should also check that the mysql functions return success (as #RocketHazmat suggests in a comment). Many return false on error.

Answer 2

mysql_query("UPDATE `users` SET `password` = '".SHA1($npassword)."' WHERE `username` = '".$_SESSION['username']."' AND `ID` = '".$_SESSION['id']."'");
You have the mysql_query set up but i dont see you executing it