Fatal error: Call to a member function isLogged() on a non-o

Hi,
I am running OC 1.5.3.1 and all of a sudden this error message is appearing;

Fatal error: Call to a member function isLogged() on a non-object in /home/healthp/public_html/admin/controller/common/header.php on line 102

No changes were made. Error simply appeared as far as I know..

I have checked and the admin config.php is correct.
I also have re-uploaded the admin folder from the default install folder

Both these have failed.

The offending line in the header.php is;

if (!$this->user->isLogged() || !isset($this->request->get['token']) || !isset($this->session->data['token']) || ($this->request->get['token'] != $this->session->data['token'])) {
$this->data['logged'] = '';

Any suggestions anybody?

Are you sure you uploaded the files into the correct folder and with the right structure? Also, make sure the config files are set up correctly with the paths to folders.. If you have not made any changes to your install the it would not hurt to re upload all files.

Hi, thanks for the reply. Yes I am certain that there were no changes made to the site. But it is a live store and was functioning correctly. This just appeared

The last time the admins config.php was modified was the 28/5/12
I have re-uploaded the Admin folder with the exception of the config.php, php.ini and index.php. I dont want to upload the whlole lot of the files as it will screw up the modifications that have already been made....or am I wrong??