Post Form data to Update MySQL get 500 Server Error - php

Question

I am trying to put together a form that is populated from data in a MySQL. Then I want to allow the visitor to change information in that form and by pressing a button, update the database with the new information. This is going to be used by only about 700 people or less to update their member information and access to the form will be through an email that the member gets, so it doesn't have to be a bullet proof system.
I am giving the member a link that displays their member information (that part works) It is just when I attempt to send them to the last page that would do the update is where I get the 500 Server Error. Here is some of my code from the form for them to update. It populates the fields from the database and allows the member to edit the field :
<form name="update" action="submitupdate.php" method="POST" />
<input type="hidden" name="id" value="<?=$record['id']?>"
<tr><b>Business Name: </b></br><input type="text" size="55" name="business_name" value="<?=$record['business_name']?>" ></tr></br></br>
Here is my submitupdate.php (That I get the 500 Server error on):
$con = mysql_connect("lxxxxx", "xxxxxx", "xxxxx");
mysql_select_db("xxxxxx", $con);
if(!$con){
die("Can not connect " . mysql_error());
}
$id = $_POST['id'];
$business_name = $_POST['business_name'];
$phone = $_POST['phone'];
$fax = $_POST['fax'];
$address1 = $_POST['address1'];
$address2 = $_POST['address2'];
$city = $_POST['city'];
$state = $_POST['state'];
$zip = $_POST['zip'];
$website = $_POST['website'];
$contact = $_POST['contact'];
$email = $_POST['email'];
if(isset($_POST['update'])) {
$UpdateQuery = "UPDATE members SET business_name='$_POST[business_name]', phone='$_POST[phone]', fax='$_POST[fax]', address1='$_POST[address1]', address2='$_POST[address2]', city='$_POST[city]', state='$_POST[state]', zip='$_POST[zip]', website='$_POST[website]', contact='$_POST[contact]', email='$_POST[email]', update_flag='$_POST[update_flag]', WHERE id='$id'";
mysql_query($UpdateQuery, $con);
$retval = mysql_query($UpdateQuery, $con);
if ($retval )
{
$sql = "SELECT * FROM members WHERE id = $id";
$my_Data = mysql_query($sql,$con);
while($record = mysql_fetch_array($my_Data)) {
?>
</br>
Then I just want to display the updated record
<tr><b>Business Name: </b></br><input type="text" size="55" name="business_name" value="<?=$record['business_name']?>" ></tr></br></br>
<tr><b>Phone: </b></br><input type="text" size="55" name="phone" value="<?=$record['phone']?>" > </tr></br></br>