HTTP 500 : really Need help solving what & where to Look

[eluser]vincej[/eluser]
Out f the blue I am getting an "internal server" error on only 1 page of my site. My ISP tell me:

"We found that the site is showing premature end of script headers: index.php, referer: http://countrylanefarms.com/admin/pos/completesalelist" - The weirdest thing is that I am not even getting an error off this controller / page .. but rather the one that follows on from it.

can someone please help me with this - I am completely lost on this - MANY THANKS !

the code for this controller is:

Code:

<?php

function completesalelist(){
$data['title'] = 'Complete Sale';
$data['completesalelist'] = $this->MPos->completesalelist();
$data['main'] = 'admin_completesale_waiting';
$this->load->vars($data);
$this->load->view('dashboard');
}
?>

code for Model:

Code:

<?php
function completesalelist(){
$sql="
SELECT `firstname`,`lastname`,`customerid`
FROM customer,confirmedorder
WHERE customer.id = confirmedorder.customerid
AND confirmedorder.picking = 'finished'
AND confirmedorder.sale = 'open'
GROUP BY customer.id
";
$Q = $this->db->query($sql);
if ($Q == FALSE) {
$this->session->set_flashdata('message', 'No Accounts Waiting');
}
elseif ($Q->num_rows()>0 ){
$completesalelist = $Q->result_array();
return $completesalelist;
}
  
else $this->session->set_flashdata('message', 'No Accounts Waiting');
}

?>